334 



MOSSES AND FERNS 



CHAP. 



cells now repeat the same divisions, and the two diverging rows 

 of inner cells form the beginning of the young veins. 



Except the smallest veins, which are collateral, the bundles 

 are typically concentric, and differ only in minor particulars 

 from those of the stem. The ground tissue of the petiole shows 

 much the same structure as that of the rhizome in most Ferns, 

 and usually develops several layers of hypodermal sclerenchyma. 

 In the lamina, the cells of the ground tissue, or mesophyll, as the 

 leaf expands, separate and form large intercellular spaces be- 



A. 



FIG. 185. Adiantum emarginatum. Development of the stomata, X525; v, accessory 

 cell; st, stoma mother cell. 



tween them. The cells are in many places connected by pro- 

 longations or protrusions of the wall. On the upper side, in 

 cases where no stomata are developed, an imperfect palisade 

 parenchyma may form, but in none of the forms examined by 

 me was it nearly so distinct as in Angiopteris. The fully-de- 

 veloped epidermal cells are very sinuous in outline, and always 

 contain numerous chloroplasts. 



In Onoclea stntthioptcris stomata are developed only upon 

 the lower side of the lamina, but sometimes these also are found 



