3B8 
THE GARDENING WORLD 
January 26, 1889. 
FORMS OF LEAVES. 
Leaves are as varied and diversified in outline as tlie 
plants that bear them, and if the argument be extended 
so as to include minor matters of detail, it would often 
he a difficult matter to find two exactly alike on the 
same plant. Assuming, however, and for all practical 
purposes it will suffice to reckon that every plant bears 
one or at most a few forms, the difficulties of enumer¬ 
ating them are considerably lessened. There are from 
100,000 to 300,000 flowering plants throughout the 
world, according to different authorities and botanists, 
who, to make themselves intelligible, have agreed upon 
a few standard types, to one or other of which they 
compare every form they meet. Under the circum¬ 
stances it is not surprising to find them disagreeing in 
many points of comparison, for it often happens that a 
leaf may, with equal right, be referred to two or more 
standard types, because not exactly like either. 
Notwithstanding the variety and complexity—not 
only in structure but in outline—of the leaves of plants 
at present clothing the earth’s surface, we have reason 
to believe they existed in much simpler forms at an 
earlier period of the earth’s history. The solid crust of 
the earth may be compared to a museum, and the 
various strata to shelves on which are piled specimens 
of the plants then existing and clothing the habitable 
surface. The record would have been complete if a 
regular gradation or series of forms had been preserved, 
but unfortunately for science there are not only 
numerous missing links in the chain, but unspannable 
gaps that can never be bridged over. Bearing this in 
mind, it would often be impossible to recognise from 
fossils, plants that make their reappearance at a later 
epoch if they had undergone much change during the 
interval of their disappearance from geological record. 
On the other hand, imperfect as they are, these 
records are valuable, and show an evident progression 
of plant life from simple forms to that of a more 
complex kind. 
Gradation of Forms. 
Further evidence of this succession or gradation of 
forms might be adduced by a study of living specimens. 
Two plants collected in widely separated localities 
might present so many differences as to be regarded as 
two perfectly distinct species.. By a careful exploration 
of the intervening districts or countries, a numerous 
series of intermediates might be found connecting 
the two extremes, and confirming their common 
parentage. 
A study of the plant from its infancy often reveals 
many successional stages, from the primary and 
usually simple leaf up to that of the greatest com¬ 
plexity occurring in the species ; and the study of its 
embryology gives more reliable evidence of its genealogy 
or ancestral line of descent than geological record itself. 
The common Passion Flower (Passiflora ecerulea) affords 
a good instance of plant evolution from the juvenile to 
the adult state. The first true leaf is small, undivided, 
and not unlike the cotyledons or seed-leaves, succeeded 
by eight to ten more that differ from the first chiefly 
in size. A series of three-lobed forms appear abruptly, 
and these, in turn, give place to five-lobed forms which 
may, under certain conditions, exhibit no further 
division. Under favourable circumstances, however, 
the leaves of the adult plant become more or less five 
to seven-lobed, with long finger-like divisions. In 
most plants the successional stages are fewer and the 
transition more abrupt. Other species of Passion 
Flower having undivided leaves in their adult state 
merely show development in size from the seedling 
upwards. The first leaf of the Horse Chestnut has five 
leaflets—a degree of evolution little behind that of the 
parent, while the seedling Laburnum has three leaflets 
and the adult no more. The explanation of these 
latter phenomena is that the offspring, through causes 
which I need not here discuss, inherits the peculiarities 
and developments of its parents at a still earlier and 
earlier age, until we find little or no trace of evolution 
in the seedling at all. 
Cause of Form. 
The agencies at work in bringing about the present 
conformation of vegetation are numerous, but the 
fundamental principle of variation underlying the whole 
is an inherent property pervading all living matter, and 
inducing it to become more and more complex and 
differentiated. Under such a principle there would be 
a constant tendency in every organism to ascend the 
scale, becoming more and more complicated and pre¬ 
sumably perfect. But there are so many minor and 
collateral influences at work, that plants do not always 
ascend the scale of organisation, but deviate one way or 
another according to external influencing conditions, 
so that there is often an apparent degeneracy in order 
to accommodate themselves to their surroundings. 
The natural outcome of this variation is designed to 
fit plants to occupy different positions on the earth’s 
surface, so that every part capable of supporting vege¬ 
tation of some sort may be clothed ; and further, the 
changing conditions, both physical and geographical, to 
which our earth is subject, necessitates the faculty in 
plants of changing their physical and external con¬ 
formation in order to exist. There is a general 
competition—sometimes severe—amongst the members 
of the vegetable kingdom for the richest and best 
places, and while the weaker give way, the stronger 
survive and occupy their places. Variation, however, 
may fit the former for some other position where the 
latter are unable to follow them. It needs but little 
argument or demonstration to show that conditions 
such as temperature and exposure to light, and sur¬ 
roundings such as mountains and other kinds of 
natural shelter, or submersion in water, all exert a 
powerful influence in modifying the forms of leaves. 
For instance, the leaves of the Water Lilies, through a 
long series of developments and evolutions, have 
become what we now see them, structures designed for 
floating, while the Oak and the Ash have developed 
in another direction, and are totally unfitted for an 
aquatic condition. 
There is every reason to believe that aquatics were 
more prevalent at a former period of the earth’s history 
than they are at present, owing to a more imperfect 
separation of land and water, and a greater abundance 
of atmospheric moisture. These aquatic plants have 
either died out, or changed their habits and become 
adapted to live on dry land. It would be an easy 
matter to point out plants adapted to fill an inter¬ 
mediate position between aquatic plants proper and 
those confined solely to dry land. Heat and moisture 
combined have undoubtedly wielded a powerful 
influence on vegetation, in shaping and moulding the 
endless variety of leaves we see around us. Witness 
the wealth of the tropics in evidence of this, where the 
two great and favourable conditions are combined, 
giving birth to the gigantic leaves of the fan Palms, 
the Victoria Regia, and hosts of other highly developed 
subjects. In the absence of moisture the vegetation 
dwindles to small-leaved, hairy or spiny forms, or gives 
place to the fleshy-leaved succulents, and other 
drought-resisting subjects. Reverting to the vegetation 
of our own comparatively sunless climate, the forms of 
leaves become much tamer and more capable of 
resisting our changeable seasons. Yet even here I 
believe our Oaks, Ashes. Limes, Maples and other 
deciduous trees originated when or where the climate 
was moister and warmer than ours is at the present 
day. 
Importance of Light. 
Light is another, and one of the most powerful 
factors, affecting not only the external conformation, 
but the internal construction of leaves. The size and 
shape of the leaves, their arrangement, the length of 
the internodes and the petioles, all bear a definite 
relation to one another in different plants, and to 
their exposure to light. If the leaves are large, the 
petioles or the internodes, or both, must be long, as in 
the Plane and Sycamore, or the leaves may be long 
and narrow, as in the Sweet Chestnut. They may, on 
the other hand, be arranged in one plane, so as to 
catch the incident rays of the sun, as in the Lime, 
Elm, or Custard Apple ; or they may be very large and 
arranged in tufts umbrella-fashion on the apex of the 
stem and branches of trees, as in the exotic Sterculias, 
Palms, Grias, and many others. So powerful is the 
influence exercised by light upon leaves, that it 
destroys the symmetry of their arrangement in 
probably hundreds of cases, making them when 
opposite, or in whorls of three or more, greatly dis¬ 
proportionate in size and unequally mated. Trees and 
shrubs, more especially those with evergreen leaves, 
have a more conspicuous and evident arrangement of 
their leaves, with regard to a perfect exposure to light, 
than herbaceous plants, owing to the necessity of 
keeping the former in a healthy condition for a longer 
period of time. The effect of insufficient light is 
readily observable in our hothouses in winter, 
especially during foggy weather, when rapidly-growing 
thin-leaved plants lose colour in a comparatively 
short space of time. Anyone may test the influence 
of light upon leaves for himself in the follow¬ 
ing manner. Of two young and growing leaves 
under like conditions, take one and enclose it in a bag 
of some dark material, so as to exclude the light, 
leaving the other fully exposed. That where the light 
is obstructed will remain small and even lose colour, 
proving that although leaves are perfectly formed 
in the bud, they never attain their natural size 
and texture, unless fully exposed to the influence of 
light. 
This is readily explainable on the grounds that the 
leaves of plants are at once a laboratory or workshop 
in which all the material is prepared for the construction 
of more leaves, their proper development, the con¬ 
struction of wood, and the production of flowers and 
fruit. This applies only to the leaves containing 
chlorophyll or leaf green.— J. F. 
(To be continued.) 
-->X<-- 
CYPRIPEDIUMS* 
Popular as Orchids have become in recent years, none 
has secured a greater share of favour than the Cypri- 
pediums, though they certainly cannot be ranked 
amongst the most brilliant of the family in floral 
colouring. In devoting a few minutes to the con¬ 
sideration of their chief characters we may gain some 
idea why they are so popular, but it would be impossible 
to deal exhaustively with the subject in a paper of this 
kind, which is intended to be suggestive, and as a 
prelude to discussion. 
General Characters. 
Orchid flowers are usually recognised without difficulty 
even by those uninitiated in horticultural or botanical 
mysteries, but no genus stands out so conspicuously 
distinct from others, yet so uniform in the principal 
characters itself, as the Cypripedium. Everyone 
knows a Cypripedium the instant a flower is seen ; 
there is not the slightest difficulty about it, no danger 
of mistaken identity. The variations are numberless, 
yet are united by certain predominant characters 
which constitute one of the most clearly defined groups 
of Orchids. It is assumed that the origin of the 
Cypripedium dates far back in the history of present 
plant life, at a time when Orchids had not advanced 
so far in their complexity of structure, and a simpler 
order of things prevailed. Darwin made a special 
point of this in his work on the Fertilisation of 
Orchids, and he remarks that “an enormous amount 
of extinction must have swept away a multitude of 
intermediate forms,” leaving the Cypripedium known 
to us isolated from all other members of its family, with 
the exception of a few obscure species that do not come 
within the horticulturist’s ken. 
The antiquated simplicity claimed for Cypripedium 
flowers is not, however, very apparent to the casual 
observer. The sepals, petals and labellum are present 
as in other Orchids ; there is also the column, com¬ 
pounded of stamens and pistil in the usual position, 
hut here a difference is noted that will furnish a key to 
the whole structure. At the top of the column, and 
just above the entrance to the lip, is a fleshy plate, 
various in size and shape, which occupies the place of 
the anther-case in most other Orchids, and being theo¬ 
retically regarded as a metamorphosed or functionless 
stamen, it takes the name of staminode. Behind this 
are two anthers, one on each side of the column, 
another divergence from the normal Orchid structure ; 
then below the staminode is a stigmatic plate projecting 
from the front of the column. The shape of the 
labellum is also exceptional, and not only gives the 
plant its botanical name, but also its popular and very 
widely accepted one of “Ladies’ Slippers,” varied in 
North America by the title Mocassin Flower, both 
referring to the pouch or slipper-like lip. Here, again, 
it may be remarked, the assumed simplicity of structure 
is by no means evident, for the labellum is widely 
different from all other Orchids, and the form is 
approached by very few flowers, the Calceolaria affording, 
perhaps, the most familiar example of any similar 
pouch-like structure. IVe can scarcely imagine how 
an organ such as the Cypripedium lip could be de¬ 
veloped even from the strange forms which prevail in 
the Orchid family without a very long series of 
changes. It is so artificial in appearance—if that term 
is admissible—and seems to be so exactly adapted for 
its particular purpose, that it is difficult to imagine it 
to represent an earlier and simpler form of structure. 
If we may indulge in such fancies, the most probable 
view is that starting from a primitive form, the Cypri¬ 
pedium has developed in an altogether different 
direction from other Orchids, hence the wide divergence 
in characters now observed, which is sought to be 
accounted for by the extinction of intermediate forms. 
In Orchids we naturally look for special adaptations 
to favour cross-fertilisation, and the labellum commonly 
serves as the chief means by which wandering insects 
*A paper by Mr. Lewis Castle, read at the Horticultural Club, 
January 15th, 1SS9. 
