144 



THE MARATTIALES 



a group of actively dividing cells which constitute its growing pomt, but no one of 

 these can be certainly named as the apical cell (fig. 117, 3)- The embryo now 

 rapidly increases in breadth until the apex is almost flat, except for the slight pro- 

 jection in the center, where are situated the stem apex and the young cotyledon. 

 The whole embryo at this stage may be described as "top-shaped." The stem and 

 cotyledon occupy a comparatively small portion of this broad termmal area and 

 are surrounded by a ring of large absorbent cells, physiologically, at least, belonging 

 to the foot and merging insensibly into the similar cells which make up the lower 

 half of the embryo. 



Fig, 118. — D.jamaicensh. 

 A, B. Two sections of a large embryo. X150. col, cotyledon; it, stem; f, foot. C. Root apex of same embryo. 



X360. 



As in the other Marattiaceae, the very young embryo shows no trace at all of 

 the root and this appears only after the embryo has reached a comparatively large 

 size. The origin of the root is exactly the same as in the other genera and it is a 

 strictly endogenous structure. The single initial cell arises deep down in the tissue 

 of the embryo, usually below the cotyledon and probably from the same octant as 

 that from which the cotyledon is formed, but owing to the displacement of the 

 original division walls this can not be determined positively and it may be that the 

 position of the root is not always exactly the same. The apical cell of the root soon 



Fig. llg. — D.jamaicensis. 

 A. Two sections of an advanced embryo. Xioo- B. Stem apex of the same embryo. X220. 



becomes conspicuous, but it does not usually show the triangular form as seen in a 

 longitudinal section, but appears more nearly square, as in Marattia. This root 

 initial cell is quite variable in form, but more commonly it appears in longitudinal 

 section with a truncate base (fig. 117, r). In transverse section it approaches the 

 triangular form, but is more or less irregular in outline. The lateral segments cut 

 off from the initial cell are large and contribute later to the root cap as well as to the 

 inner root tissues, and it is not impossible (as Farmer believes to be the case also 



