II4 OUTLINES OF PLANT LIFE. 



finally wrenched off by winter storms, or pushed off in the 

 spring by the developing buds beneath them. 



EXERCISE XIX. 



Leaves. — Examine the forms, branching, and venation of such leaves 

 as can be secured. Unfolding buds show modes in which leaves are 

 folded or rolled. Special directions for study seem unnecessary. A 

 demonstration of the structure of a lily or lilac leaf (T[ 136-139) is 

 desirable. (For flower leaves see p. 210.) 



143. Summary. — The form, structure, and position of 

 foliage leaves are chiefly dependent upon the amount and 

 direction of light. The first leaf or leaves of the embryo are 

 usually transient; even secondary leaves rarely live more 

 than a single season. They arise in regular succession on 

 the stem and at such points as are least crowded. The parts 

 of a leaf are blade, base, and stalk ; any one or two may be 

 wanting. The base is often sheathing or branched to form 

 stipules. The stalk may be winged to act as a blade. The 

 blade is in one piece or more or less branched into lobes or 

 into leaflets. The veins, containing vascular strands, supply 

 all parts with water, and when strong prevent tearing. The 

 leaf rudiment, instead of developing into a foliage leaf, may 

 form a pitcher, a tendril, a thorn, a scale, a flower leaf, a 

 storage place, etc. The internal spaces of the leaf connect 

 with the air through stomata, which are guarded by a pair of 

 valve-like cells. These by changing form can regulate the 

 evaporation of water from the leaf, and also permit ready 

 entrance of air. Leaves often live over winter in a rudimen- 

 tary condition in the bud. They fall usually because of the 

 formation of a separation layer of cells across the leaf base. 



