26 PART I. ORGANOGRAPHY. 



modified into a tendril for the purpose of enabling the plant to 

 climb. 



In most of the Australian Acacias, trees which in other 

 countries are noted for their graceful, feathery foliage, both blade 

 and stipules of all but the earlier leaves fail to develop, while 

 the petiole becomes flattened and performs the functions of a 

 blade. These flattened petioles, or phyllodia, as they are called, 

 are simple, parallel-veined, and placed with their edges vertical ; 

 hence the Australian species present a widely different appear- 

 ance from the nearly related ones of other countries. Fig. 51 

 represents one of the earlier leaves of one of these Acacias, 

 showing a widened petiole, and the tendency to develop phyllo- 

 dia, and Fig. 52 represents a fully formed phyllodium of the same 

 tree. 



It is frequently the case, however, that all parts may be 

 present, but, owing to a change in their usual form, or a partial 

 coalescence with other parts, the presence of one or the other of 

 them may be more or less obscured. In grasses, for example, 

 the stipules appear to be united with the petiole to form the 

 sheath which clasps or encloses the stem, but usually their 

 apices are free and slightly project at the junction of the blade 

 and sheath, forming what is called the ligule, Fig. 53. 



In the Polygonums the stipules co- 

 here with each other, and form a sheath 

 about the stem. This stipular sheath, 

 which is usually membranous, and 

 may or may not have a free portion, is 

 called the ochrea. Not infrequently 

 also the stipules become adnate to the 

 petiole, as in the Rose, Fig. 54, or 

 become converted into spines, as in 

 the Locust, Fig. 55, and in some in- 

 stances they are changed into tendrils, 

 as in the Green Briar, Fig. 56. In 

 f,g. S3.- Leaf of grass, Zl- many instances also they are scaly, and 



ing lamina, sheath and ligule. a f ter serv i n g the piirposeof protec- 



tion in the bud, fall away when the leaves expand. 



Considering the structure of the parts of the leaf, we find 

 the blade and stipules, when normally developed, consist of a 



