LEAF STRUCTURE. 



171 



A cross-section of a Fern stem shows a mass of 

 parenchyma, supported by an outer sheath or tube of 

 vascular-woody bundles, the whole 

 inclosed by a cortex of dense scleren- 

 chyma, the leaf stems presenting the 

 same structure (Fig. 525). 



424. Tissues of Bryophyta, moss- 

 like plants. The higher types of this 

 division, while largely made up of 

 cellular masses, have a semi-vascu- 

 lar-fibro arrangement, and in some 



525 



525, Section of an Acrogenous 

 stem of Tree-Fern (Cyathea), 



mosses the fibers are so strong as to showing the vascular bundles 



imbedded near the circumference 

 of the cellular mass. 



approach a woody character. 



CHAPTER V. 



LEAF STRUCTURE. 



425. Leaves are composed of the same general 

 structure as the stems and branches which they clothe 

 and adorn, and are made up of vessels and tissues 

 already described: 1, woody fiber, which constitutes 

 the frame-work ; 2, cellular tissue, which fills up the 

 spaces between the ribs or frame-work formed by the 

 woody part. The leaf of a Maple, Elm, or Apple is 

 composed of: 1, the leaf-stalk, by which it is attached 

 to the stem or branch; 2, the blade, the expanded 

 part. The leaf-stalk or petiole is a column of bundles 

 of woody fiber and green tissue, covered by the epi- 

 dermal tissue. These bundles extend in length to suit 

 the size of the blade, throwing off branches and 

 branchlets to construct the frame, making an irregular 

 net-work, the meshes of which are filled up by the 



