MIXUTE STRUCTURE OF THE LEAF. 



13 



early in spring; others need more heat and start slowly. To a 

 limited extent, the less moisture plants contain, the more cold 

 they "will endure withoiit injury. When green leaves are exposed 

 to severe cold, if the thawing be gradual, in many cases they will 

 not be injured, but some plants quickly perish with frost, no 

 matter how slowly it is removed. 



Minute Structure of the Leaf. — The blade is traversed longi- 

 tudinally hj fihro-vascular hiiiuUes, which may be distinguished 

 'AS priinaries, those the most complete, and those less complete, 

 as secondaries and icrtiaries. The bundle is reiinforced by a 

 nerve on the upper side of the leaf. That in the middle of the 

 leaf is usually the largest, and is called the mid-vein, mid-rib, 

 or keel. 



At the base of a broad leaf, such as that of Indian corn, there 

 is a large concave mid-rib, which contains many fibro-vascular 

 bundles. Following the mid-rib towards the apex of the leaf, 

 we shall see that one after another of these fibro-vascttlar bundles 

 leaves the mid-rib and passes into the blade. The outer bundles 

 in the lower part disappear in the margin of the leaf, the central 

 ones only, extending to the apex. 



Fig. 11.— Section of a leaf of Andro- 

 pognn (foiffffn/w, where the whole blade 

 is rocliiced to what answer.s to the chan- 

 nelled niul-rib of Zca maus. 1x35. — 

 (Duval-Jouve.) 



The blade of a leaf of Poa jira- 

 ril'2i^l-^^f^^Z^'^f'T.X^%tensis (June grass) and others, have 

 worthj '''^"'''''"- •^''l'^--'*"'^- veins, which are exactly parallel. 



