512 
NATURE 
[March 29, 1883 
that special selective causes (protection against creeping 
imsects, &c.) have often come into play in preserving or 
modifying such decurrent wings, stem-lines, auricles, 
clasping stipules, and rows of hairs ; but as a whole they 
nevertheless point back distinctly to the origin of dicotyle- 
donous stems from superposition of leaves and midribs 
upon one another. They are rudimentary forms of stem- 
lamina. 
Sessile leaves are particularly apt to be lanceolate. 
They approach nearest among dicotvledons to the mono- 
cotyledonous type. The botanist will readily fill in 
examples for himself. 
Fic. 34.—White Deadnettle (Lamium album). 
On the other hand, it is clear that the conditions under 
which leaves assume the orbicular and peltate types can 
only occur where there is least subordination to a central 
stem. And these conditions must have occurred for im- 
mense numbers of generations in order to overcome the 
ancestral tendency towards the lanceolate or ovate form. 
For a leaf must first pass through a cordate or reniform | 
stage, like that of the coltsfoots, before it can reach an 
orbicular shape, like that of our common waterlily ; and 
even when it becomes completely circular, like the Victoria 
regia, it may still retain a mark of junction where the | 
Fic. 35-—Lime. 
overlapping edges have met without becoming connate. 
In the case of Victoria regia the transformation has been 
traced during germination. The first leaves produced by 
the young plant are linear and submerged ; the next are 
sagittate and hastate ; the later ones become rounded, 
cordate, and orbicular; and even when they assume the 
peltate form, the line still marks the point of union. 
This sufficiently accounts for the rarity of perfectly peltate 
leaves, such as those of Zrof@olum, Hydrocotyle, and 
Podophyllum. Radical leaves growing on long footstalks 
will be oftenest orbicular cordate; stem-leaves on the 
same plant may pass from ovate-cordate to ovate, lanceo- 
late, and linear. Large cordate radical leaves will be 
most frequently produced from perennials with richly- 
stored rootstocks. The sagittate and pointed leaves of 
Arum and Sagittaria show the furthest step attained in 
the same direction by monocotyledonous foliage, starting 
from the liliaceous form. 
Where the stem, or, what comes practically to the same 
thing, solitary ascending branches, rise high into the air, 
especially with opposite leaves, we get a common type 
which may be well represented by the white deadnettle 
a RA 
(Fig. 34). Hedgerow plants with perennial stocks fre- 
quently assume this type. It reappears almost identically, 
under the very same conditions, in so distant a group as 
the true nettles ; and though it is possible that the causes 
which produce mimicry in the animal world may here 
have come somewhat into play, so as to modify sundry 
Lamiums into the similitude of the protected Urtica, yet 
the analogy of other Labiates shows that the circumstances 
alone have much to do with producing the resemblance. 
For a great many tall-stemmed hedgerow Labiates closely 
Fics. 36 and 37.—Begonias. 
Fic 38.—Cow-parsnip. 
approximate to the same type: for example, Lammzum 
| galeobdolon, Ballota nigra, Galeopsis tetrahit, Stachys 
stlvatica, and S. palustizs. Compare, mutatis mutandis 
for ancestral peculiarities, the other hedgerow plants, 
Scrophularia nodosa and Alliaria officinalis. On the 
other hand, notice the orbicular long-stalked lower leaves 
of the latter (especially when biennial) side by side with 
the lower leaves of some Labiates, such as Wefela 
glechoma. Indeed, the Labiates as a whole present an 
excellent study of local modification in an ancestral type, 
Fic. 39.—Creeping leaves of ivy. 
according to habit and habitat. Take as other groups of 
this family the following: first, Mentha and Lycopus ; 
then, Salvia pratensis, Prunella, Marrubium, radical 
leaves of Ajuga reptans; and lower leaves of Vefeta 
glechoma; finatly, the typical form dwarfed in little 
prostrate retrograde types, such as Thymus serpyllum 
and Mentha pulegium. Compare these last with other 
prostrate or dwarfed types elsewhere, like Veronica ser- 
pyllifolia, Peplis portula, Hypericum humifusum, Montia 
Jontana, and Arenaria serpyllifolia. 
As grassy types, the best familiar examples are those 
