326 AfOSSES AND FERNS chap. 



growth in the outer cells of the young segment, which thus form 

 a lobe. The marginal cells divide rapidly by longitudinal walls, 

 so that at first the young pinna does not grow from a single 

 apical cell, but sometimes two of the division walls intersect and 

 an apical cell is formed. Whether this always happens could 

 not be absolutely determined. As each pinna corresponds to 

 a segment of the apical cell of the leaf, it follows that they 

 alternate with each other on opposite sides of the rachis. 

 Where these grow from an apical cell, the divisions follow 

 those in the apex of the leaf. From the inner cells of the 

 segments the rachis of the pinna is developed. The midrib of 

 each lobe of the pinna bears the same relation to it that the 

 rachis does to the pinna itself. The secondary veins arise in 

 acropetal succession, and at first form a strand of procambium 

 reaching from the midrib to the margin. Where dichotomy of 

 the veins occurs, as it so frequently does in their ends, this is 

 connected with a dichotomy of the marginal group of meriste- 

 matic cells.^ Each marginal cell, like the segment of the apical 

 cell of the leaf, divides into an inner and an outer cell. The 

 latter then divides longitudinally, and the dichotomy is thus 

 inaugurated. These secondary marginal 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 in 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 scler- 

 enchymas. In the lamina, the cells of the ground tissue, as the 

 leaf expands, separate and form large intercellular spaces 

 between them. The cells are in many places connected by 

 prolongations 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- 

 developed epidermal cells are very sinuous in outline, and 

 always contain numerous chloroplasts. 



In Onodea struthiopteris stomata are developed only upon 

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

 upon the upper surface as well. Usually, but not always, the 



^ Sadebeck (6), p. 27c. 



