54 THE FLOWERING PLANT. 
trees cast no shadow, for the phyllodes are vertically expanded. 
Several considerations show that the structures in question are 
actually petioles. An ordinary acacia leaf is complex in shape, 
and there is one species, Acacia melanoxylon, which possesses 
both phyllodes and ordinary leaves. Not only so, but upon the 
same tree numerous gradations 
are found between the two, from 
examples with well-developed 
laminee and slightly flattened 
stalks, down to others with much 
reduced laminz and _ phyllode- 
like stalks. Again, although a 
mature acacia may possess no- 
thing but phyllodes, yetthe early 
leaves of the seedling are. nor- 
-mal. These are succeeded by 
others which are more and more 
phyllode-like, till, finally, only 
phyllodes are produced (cf. 
fig. 12). 
A number of plants are now 
known which are carnivorous or 
insectivorous, that is, they attract 
insects by various devices, and 
make use of them as food. The 
pitcher-plants are well-known 
examples. The ‘“ pitchers” are 
Fig. 12.—Bipinnate Leaf of Acacia, with hollow leaf structures, and, in 
ini tenure the North American Sarrace- 
nias, belong mainly to the petiole, though a small hood-like 
lamina is present which overhangs the mouth of the pitcher. 
The surface of the petiole may be glabrous, or else provided 
with hairs or emergences. 
We now come to the Lamina. This is generally expanded 
horizontally, so that in shoots which grow upwards the upper side 
is turned towards the stem. The twigs of many trees, as, e.g., the 
beech, elm, and yew, are more or less horizontally directed, and 
their leaves are twisted upwards so as to retain their normal 
position with regard to the light. Bilateral shoots are the result. 
The same thing occurs in many creeping, trailing, and climbing 
forms, of which periwinkle (Vinca) and ivy may be taken as 
examples. The bilateral arrangement is generally limited to 
alternate leaves. Opposite leaves under the same circumstances 
would overlap. The horizontal position is not, however, assumed 
by all leaves. Some of them are vertical, like the phyllodes of 
acacias, and in this case the same end, ¢.e., protection from a 
YY 
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