MINUTE STRUCTURE OF THE LEAF. 



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

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

 they will endure without 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 by fibro-vascular bundles, which may be distinguished 

 as primaries, those the most complete, and those less complete, 

 as secondaries and tertiaries. The bundle is reenforced 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-vascular 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. 



us. - ( Sud- 



FIG. 11. Section of a leaf of Andro- 

 pogon lanigerum, where the whole blade 

 is reduced to what answers to the chan- 

 nelled mid-rib of Zea mays. 1x35. 

 (Duval-Jouve.) 



The blade of a leaf of Poa pra- 



( June g rass > and others > hav& 

 veingj w hich are exactly parallel, 



