120 



l II 1 MAR VTT1 \l.l s 



equal parts (fig. 87, A). A rhizoid may be an of! either before or after this first 

 transverse \\ ;< 1 1 is developed, but frequently no rhizoids are formed until a much 

 later period, as in Ophioglossum. I hi- primar] rhizoid, when present, is formed 

 much as in the typical ferns, the papilla from which it develops being cut off from 

 the largei cell, and it contains little 01 no chlorophyll. I- ach of the primary prothal- 

 lial cells divides, in typical casts, h\ a longitudinal wall, so that the young gameto- 

 phyte consists of four cells, arising quadrant-wise ( fi<;. XX, A), and closely resembling 

 corresponding stages 111 Ophioglossum, except for the absence of chlorophyll in the 

 latter. Where the young plants are crowded or light is deficient, as for instance 

 when the germination occurs within the sporangium, then- is a tendency to the 

 development of a filament, a phenomenon often met with also in the typical ferns. 

 Usually one of the upper pan of cells in the four-celled stage assumes the role 

 of an apical cell, and for some time, as in the typical ferns, there is growth from a 

 two-sided apical cell (fig. XX, C). As soon as the apical cell is established, it »rows 



I 1,.. s 





\. Two germinating spores ol Marattia frax inea Smith. ■ 200. The remains oi the spore meml 



li. Young gametophyte ol ame 1 -;. (A, B, after Jonkmann). 



i . Marattia tambucina Blume. X r.c. 



I) II. M.douglasii Baker. 1)1', . 1.,-; 1; , H, x-,. G,H, show the young sporophytc. 



I) and E represent the same prothallium aftei an interval of about .1 pear. F 1^ tin- ventral riew of E. k, adventitious bud. 



very much as it does in such a liverwort as Aneura and produces a thallus of the 

 same form and structure. But as the prothallium grows older a periclinal wall 

 is formed in the apical cell, and in the outermost of the two cells thus produced 

 there is a longitudinal wall dividing it into two equal cells, and from this time on 

 it is impossible to recognize a single apical cell in the prothallium, the apex of which 

 is occupied by a group of apparently similar marginal initial cells. 



At hist tin- prothallium has a spatulate form, but before the single apical cell 

 is replaced by the group of marginal initials the outer cells of the younger segments 

 grow more rapidly than the inner ones, so that they project beyond the apical cell, 

 which thus comes to he in a depression between the two lobes, and the familiar 

 heart-shaped form so commonly found in the prothallium of most ferns is established . 

 1 he marginal initial cells vary in number with the width of the depression in which 

 they In. In a horizontal section they appear oblong in form, but in the vertical 

 sections made they have a semicircular outline (fig. XX, D, E). 



