72 THE LEAF 



compound, then note the presence or absence of stipules and petiole, after 

 which describe their form, margin, apex, and surface. 



3. Modified Leaves. — Structures are often met with upon 

 plants wliich altliough they do not possess all the parts of a 

 foliage leaf as just described, are, nevertheless to be regarded 

 as leaves on account of their origin and position upon the plant, 

 and also by the fact that they frequently bear buds in their 

 axils, and under some circumstances may become changed into 

 ordinary green leaves. Several of these modified leaves 

 receive special names as indicated below, according to their 

 position upon the stem, or according to their texture, colour, 

 and other peculiarities. 



{a) Cotyledons or seed-leaves. — These are the first leaves which 

 a flowering plant possesses, and are nearly always simple and 

 entire, and without stipules. 



Some coniferous trees (pines and firs) have seedlings with 

 several cotyledons, but dicotyledons usually possess only two 

 (Figs. 5, 103, no), while in monocotyledonous plants only 

 one is present. 



In the bean, pea, and vetch they serve merely as storehouses 

 for the food upon which the seedling depends for its early 

 growth. In the cereals and grasses generally, the chief work 

 of the cotyledon is to absorb the endosperm of the seed, and 

 transfer it to the growing-points of the young root and shoot ; 

 while in the turnip, mangel (Fig. no) and many other plants they 

 come above ground and carry on the work of 'assimilation,' 

 thus behaving as ordinary foliage-leaves. 



{b) Scales. — These are usually thin membranous leaf-struc- 

 tures, generally brown, white, or yellowish in colour, and may 

 be either complete leaves, or merely the sheaths and stipules of 

 leaves the blades of which have not developed. 



On the stems above ground they are often present as coverings 

 to the buds of trees and shrubs, acting as a protection for the 

 interior of the bud against frost, heat, rain, and the attacks of 



