March 29, 1883 | 
NAROKE 
aKS 
of the flaxes, Ste//aria graminea, Toadflax, Bastard Toad- 
flax, &c.; all of which have been largely influenced by 
monocotyledonous competition. Even a pea, Lathyrus 
nissolia, has got rid under such circumstances of its 
leaflets, and has flattened its petiole into a grass-like 
blade. Intermediate forms occur in Southern Europe. 
The peas, indeed, are papilionaceous plants which have 
largely cast off their ancestral leaf-type, in order to avail 
themselves of new conditions. JZ. afhaca has lost its 
leaflets, and flattened and enlarged its stipules so as to 
resemble simple opposite leaves; and Z. /irsutus and 
pratensis have reduced the leaflets to one long almost 
Fic. 40.—Ascending leaves of ivy. 
linear pair. Marshy plants have also often been forced 
into adopting grass-like forms. The great spearwort 
is a swampy buttercup, whose ancestral leaf has been 
lengthened ont into a long ribbon, with almost parallel 
ribs ; the lesser spearwort shows the same tendency to a 
less degree, still retaining ovate lower leaves, with lanceo- 
late upper ones; and Veronica scutellata is a similar 
marshy case among the Scrophularinez. 
When the tree-like form is attained, or free access to 
air is otherwise gained (as by climbers), the supply of 
carbon, being practically unlimited, becomes relatively 
little important, and the supply of sunlight assumes the 
Fis. 41.—Sundew. 
first place in the economy of the plant. Under such 
conditions, the great object must be to prevent the leaves 
from overshadowing one another. Now this result may 
be obtained in a great number of ways, and we must not 
expect that every tree or shrub will solve the problem for | 
itself in exactly the same fashion. 
shape into which the ancestral form is finally modified 
should sufficiently answer the purpose in view. 
matter of fact, the suitability of the actual forms and 
arrangements of tree-leaves to the functions they have to 
perform can be readily tested by observing any tree in 
bright sunshine. Onthe one hand, almost every leaf is in 
As a_ 
It is enough that the | 
full illumination, no leaf unnecessarily shading its neigh- 
bour; and on the other hand, there is hardly any interspace 
between the leaves, as may be seen by the fact that the 
shadow thrown by the tree as a whole is almost perfectly 
continuous. In short, there is no waste of chlorophyll, 
and there is no waste of sunshine. 
Mr. Herbert Spencer has called attention to the results 
of varying exposure to light in the various parts of the 
same leaves, which often causes them to become unequally 
developed. In the lime (Fig. 35) such obliquity is normal. 
In the various Begonias (Figs. 36 and 37) the resulting 
asymmetry is very noticeable. In the cow-parsnip (Fig. 
38) it is the leaflets of the same leaf which are asymme- 
trically developed, so as not to overshadow one another. 
In more symmetrical leaves, there is an equal provision 
for preventing overshadowing, only here it takes the form 
of indentation of the edge, as in the oak, or of subdivision 
into leaflets, as in the horsechestnut. In the latter case, 
indeed, the two outermost leaflets are habitually asym- 
metrical. On the whole, however, the mass of forest 
trees in temperate climates have almost entire leaves ; 
and full exposure to sunlight is secured rather by their 
special specific arrangement at the end of the minor 
branches. Most often they are more or less ovate, as in 
the elm, beech, alder, birch, and poplar. Where the 
Fic. 42.—D.onza. 
leaves are divided, the separate leaflets assume the ap- 
pearance of almost entire leaves ; compare the leaflet of 
the horse chestnut with the leaf of the true chestnut; the 
leaflet of the ash with the leaf of the hornbeam; the 
leaflet of the walnut with the leaf of the beech ; and the 
leaflet of the mountain ash with the leaf of the black- 
thorn. In all these cases, almost identical results are 
practically produced in the end by similar circumstances 
acting upon wholly unlike original types. 
Some minor typical forms exist in certain groups of 
climbers, which are worth a moment’s notice. Take as 
an example the creeping leaves of ivy. As Jong as this 
plant grows close to a wall or the trunk of a tree it 
assumes the well-known shape shown in Fig. 39. But 
‘as soon as it branches out its flowering sprays into the 
, open, acquiring a tree-like habit, which it often does on 
the top of a wall, it takes a simpler and totally different 
form of leaf, as shown in Fig. 40, growing on the same 
plant. This last type is quite comparable to that of the 
pomegranate. That both types admirably suit their par- 
, ticular situation can easily be seen by noting how well 
they fit in with one another without overshadowing. It 
would be difficult to point out the geometrical grounds 
for this relation, but the relation itself becomes obvious 
on watching an ivy-plant in broad sunshine. Moreover, 
the first or truly ivy-like form of leaf tends to recur among 
