THE AMERICAN BOTANIST. 25 



atmosphere and by means of sunlight combined to form 

 the substances found in its tissues. These combina- 

 tions are effected in the green substance of the leaf, there- 

 fore all plants that elaborate their own food must have 

 either green leaves or something to take their place. This 

 is plain in the cacti and scouring rushes (Equisetum) both 

 of which have functionless leaves and so must rely upon 

 the green in the stems for elaborating their food. 



It must not be supposed, however, that the leaf is 

 inerely the expanded green portion, or blade. This is the 

 most important part, it is true, but there is also frequently 

 to be seen a leaf-stalk or petiole and at the base of the 

 petiole where it joins the stem two little leafy or scale-like 

 objects called stipules. In many plants the stipules are 

 not present and in some they are so changed as to scarcely 

 be recognizable. In the smartw^eed family {Polygonum), 

 the papery sheaths that encircle the stem where the leaves 

 join it are derived from stipules. Sometimes the petioles, 

 also, are wanting in which case the leaves are said to be 

 sessile. 



The shape of the leaves and their arrangement on the 

 stem plaj^s an important part in the identification of spe- 

 cies. Fortunately the terms used in describing leaves are 

 in common use. Such words as round, oblong, ovate, 

 lanceolate and linear mean the same w^hen applied to 

 leaves as they do applied to other objects. Oblanceolate 

 and obovate mean simply the reverse of lanceolate and 

 ovate ; that is, in ovate or lanceolate leaves the blade is 

 broadest near the base where it joins the petiole, while in 

 the others it is broadest at the apex. If the tip of the leaf 

 is rounded it is o/jtuse; if pointed, it is acui:e; if the tip is 

 long and suddenly narrowed, it is acuminate; if it ends 

 in a hard point it is cuspidate or mucronate. The base of 

 the leaf also has names to distinguish its outlines. If the 

 margins curve inward to join the petiole the base is cor- 

 date as in the morning glory ; if these basal lobes are long 

 and pointed it is sagittate as in the arrow-head {Sagit- 

 taria) ; if the lobes are at right angles to the rest of the 



