PTERIDOPHYTA 253 



In the foot, the apical growth is of very brief duration, and the 

 divisions do not show any definite succession. 



Root. In the root-quadrant, the larger of the two octants at once 

 assumes the role of an apical cell, the smaller one undergoing very 

 little further growth. After one or two series of lateral segments, 

 apericlinal wall cuts off the first cell of the root-cap, and thence- 

 forth there are four series of segments, as in the roots of the mature 

 sporophyte. 



Cotyledon. The primary leaf or cotyledon behaves at first much 

 like the root. One of the octants develops faster than the other, 

 and the growth is also apical ; but, of course, no cap-cells are 

 formed, and later the tetrahedral apical cell is replaced by a two- 

 sided one, and the leaf begins to assume its characteristic flattened 

 form. 



The establishment of the separate growing-points in the embryo 

 soon causes the growing organs to project, and the embryo loses 

 the globular form found in the early stages. Growth is rapid in both 

 root and leaf, and these presently break through the overlying tissue 

 of the gametophyte. The root turns downward and penetrates the 

 earth, and the leaf, expanding and growing upward, spreads its flat- 

 tened blade to the light-rays. So soon as this is accomplished, the 

 young sporophyte, which has hitherto obtained its nourishment from 

 the gametophyte by means of the foot, 

 now is completely independent, and 

 the gametophyte soon dies, leaving the 

 sporophyte rooted in the ground. 



The cotyledon in most Ferns is fan- 

 shaped (Fig. 217), due to an early 

 dichotomy of the apex, which is re- 

 peated several times. 



Of the two stem-octants, one becomes 

 at once the tetrahedral apical cell of 

 the permanent stem, the other develops 

 into the second leaf, whose growth is 

 much like that of the cotyledon, but 

 which becomes larger. 



During its early growth, the embryo Fl( L 2l7.-0*munda Claytoniana. 

 ' , _ , . ,. ' Young sporophyte attached to 



is protected by the enveloping tissue the gametophyte ( x 6) . 

 of the archegonium venter, which 

 forms a calyptra like that found in the Bryophytes. 



Tissues of the Embryo. The young sporophyte is composed of perfectly 

 uniform parenchyma, but as the organs develop, there soon becomes evident a 

 separation of the tissue elements into definite tissue systems. A single layer of 

 epidermal cells is generally evident at an early period, and somewhat later the 

 axis of each of the primary organs shows a strand of elongated cells, especially 



