THK EMBRYO 



145 



in Angiopteris) that sometimes more than one of these terminal cells function as 

 the apical cell. 



As soon as the root apex is established its growth is very rapid and, as in the 

 other genera, there is a rapid elongation of the whole embryo, whose vertical diameter 

 very soon becomes greater than the transverse diameter, and ultimately the whole 

 embryo becomes very much elongated. In the axial region of the embryo just below 

 the stem apex there is a marked elongation of the central cells which might at first 

 sight be taken to represent the central vascular cylinder, but these elongated cells do 

 not give rise to vascular elements, but remain as elongated parenchyma and belong 

 really to the pith. The part of the foot which lies below the growing root apex 

 acts as a very massive root cap and is pushed down with the growth of the root until 

 the latter emerges from the lower side of the prothalhum. The cotyledon in the 

 meantime grows actively upward and finally penetrates the prothallium, emerging 



Fig. 120. — D. jamatcrruis. 

 Five of a series < f transverse sections from an advanced embryo. X200. I passes through stem apex; 5 shows apex of root . 



on the upper side. The growth of the stem is slight and the cotyledon and root form 

 nearly a straight line, as in the other Marattiacea? and in Op/uoglossum, so that the 

 young sporophyte may be described as bipolar. The equatorial region is surrounded 

 by the large absorbent cells in contact with the prothallium and all of these may be 

 said to function as the foot, although it is impossible to say how much of this tissue 

 is derived from the original foot. 



Til 



\N\1()MV AND HISTOLOGY OK THE YOUNG SPOROPHYTE. 



The young sporophyte, at the time the root emerges from the prothallium, is 

 very much alike in all of the genera (figs. 108, ill, I2l). The cotyledon at this 

 stage is a thick, conical protuberance, strongly curved inward over the stem apex, 

 owing to the more rapid growth upon its outer side. Close to its base lies the 

 very limited growing region of the stem, adjacent to which is the rudiment of the 

 second leaf, which at this time projects very slightly above the level of the stem apex. 

 The cells of" the latter, and also those of the young leaves, are evidently actively 

 growing cells with conspicuous nuclei and are quite different in appearance from the 

 large, transparent cells that compose the enlarged mid-region of the young sporo- 

 phyte. This is largely made up of the original tissue of the foot, but this tissue merges 

 insensibly into the basal region of the cotyledon and the upper part of the root, 

 these two organs forming in their growth almost a straight line. \\ hether or not 

 we term this middle region of the young plant the "stem," it must be borne in 

 mind that it does not arise from the activity of the extremely limited meristem, 

 forming the true stem apex. The young sporophyte is traversed by a single conspic- 

 uous vascular bundle which extends through the cotyledon and mot without inter- 



10 



