68 



A TEXTBOOK OF BOTANY 



[Ch. Ill, 8 



stood, but it seems connected with a tendency of the chloren- 

 chyma to collect more closely towards the main veins. The 

 lobing carried farther leads to compounding, which therefore 

 is either pinnate, as in Acacia, or palmate, as in Virginia 

 Creeper (Fig. 37) ; and often the leaflets are themselves 

 compounded, even more than once, as in some Ferns. 

 Parallel-veined leaves are rarely lobed or compounded, their 

 mode of venation being obviously unfavorable thereto. 

 The number of leaflets in a compound leaf can be very great, 

 or no more than three, as in Poison Ivy, or even only one, as 

 in Orange. 



Leaves differ also in the character of their margins, which 

 in some, e.g. Rubber Plant, and most parallel-veined kinds, 



are unbroken or entire, 

 but in others are sharp- 

 toothed or serrate, e.g. 

 Rose, and in others yet 

 otherwise formed (Fig. 

 38). The differences 

 seem to have no' func- 

 tional significance, but 

 represent structural ex- 

 pressions of the various 

 ways in which the chlo- 

 renchyma is arranged with respect to the vein endings. 



Leaves also display some peculiar forms of tips and bases 

 (Fig. 39). The prolonged slender tip found in some leaves 

 of tropical plants has been claimed to act as a "drip point," 

 effective in removing water from the leaf after rain, thus pre- 

 venting a long closure of the stomata; but the evidence is 

 not clear. Some leaves have the base of the blade prolonged 

 into ear-shaped (auriculate) or pointed forms, occasionally 

 making the leaf arrow-shaped. In some kinds these ex- 

 tensions grow together around the stem, which accordingly 

 seems to pierce the blade (perfoliate), while in others two 

 opposite leaves grow together in similar manner surrounding 



Fig. 38. — Forms of leaf margins. 

 (After Gray.) 



