WILSON: ANATOMY OP ALFALFA. 297 



The trichomes of the leaf are two-celled structures (fig. 15), 

 the basal cell being in the same plane with the epidermis, and 

 the outer cell greatly elongated. These trichomes are similar 

 to the most numerous type on the stem. The elongated portion 

 is marked on the margin by numerous papillse-like swellings 

 (fig. 15, c). Solereder says concerning such markings: "Ac- 

 cording to Vogelsberger, in certain species of Ebenits the end 

 cell of the trichome under discussion is provided with small, 

 tooth-like papillae, which are directed towards the apex of the 

 hair." Similar structures were observed by Schenck in Medi- 

 cago arhorea. In alfalfa the projections are directed at right 

 angles to the surface rather than towards the apex. It is of in- 

 terest to note here that the variety of alfalfa most resistant to 

 drought, the Arabian, is the most hairy economic variety. The 

 average length of alfalfa hairs is about .7 mm., the diameter 

 about .01 mm., the number of hairs to a sq. mm. being about 

 24. The ordinary observer would hardly consider the. alfalfa 

 leaf a hairy one. 



Exceptions to the general irregularity of epidermal cell 

 arrangement are found under the midrib and at the bases of 

 the trichomes. Under the midrib the cells are elongated par- 

 allel with it ; they are narrow and strongly cutinized, as well as 

 roughened on the outer wall. No stomata are found here, the 

 general arrangement apparently serving to strengthen the 

 midrib of the leaf, and also being in harmony with its con- 

 ductive function. At the bases of the trichomes the epidermal 

 cells form rosette patterns (fig. 15), with the basal cell of the 

 trichomes as a center. 



The stomata of the leaf are like those of the stem, but more 

 numerous, in places as many as 700 to a sq. mm. being found. 

 The exceptionally high number is to some extent an indication 

 of the leaf's exceptional functional activity. 



Under the stomata are extensive air spaces (fig. 22, e and /) . 

 The palisade layer (fig. 22, g) is also well aerated by inter- 

 cellular spaces. 



Under the palisade tissue, and extending to the lower epi- 

 dermis, is the spongy parenchyma (fig. 22, h) . The loose 

 arrangement of thin-walled cells containing chloroplasts is 

 typical of this region in leaves. Air spaces in this region com- 

 municate freely with the numerous stomata on the under side 

 of the leaf. 



2-Univ. Sci. Bull.. Vol. VH. No. 17. 



