MINUTE STRUCTURE OF THE LEAF. 



1» 



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

 limited extent, the less moisture plants contain, tlio 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 he injured, hut some plants quickly perish with frost, no 

 matter how slowly it is removed. 



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

 tudinally hy fibro-vascular iundlcs, which may be distinguished 

 ixsj)rimaries, those the most rr^mplete, and those less complete 

 as secondaries and ferfian'es. The bundle is reenforced l)y 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-ril), 

 or keel. 



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

 is a largo 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 bund es 

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

 ones only extending to the apex. 



Fio. 11.— Section of a leaf of Atulro- 

 viiyiin /(un'yt'nim, where tlie whole blade 

 IS reduced to what answerHto tlie chan- 

 nelled niid-ril) of Zcamays. 1x35.— 

 (Duval-Jouve.) 



The blade of a leaf of Poa pra- 

 ,.ih'^f^?-~**®""t?''^"*J'"T"l*'"f''^;' /«''<«'>■ (June grass) and others, liave 



rib of some portions of the leat ot ' \ & / ' 



^wtiu ''''""'*"'• 1 " l'5.-(.sud-^.gjjjg ^vhich are exactly parallel. 



