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 

 lamina) and slightly flattened 

 stalks, down to others with much 

 reduced laminae and phyllode- 

 like stalks. Again, although a 

 mature acacia may possess no- 

 thing but phyllodes, yet the 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 

 insect Ivor ous, 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 

 hollow leaf structures, and, in 

 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 some end, i.e., protection from a 



fig 



Bipinnate Leaf of Acaci 

 flattened petiole. 



