PTEBIDOPHYTE8. 105 



right and left (Fig. 66, Z>, E, a), the apical cell growing to its original 

 dimensions after each pair of segments is cut off. The segments divide 

 by vertical walls in various directions so that the young plant rapidly 

 assumes the form of a flat plate of cells attached to the ground by root 

 hairs developed from the lower surfaces of the cells, and sometimes from 

 the marginal ones. As the division walls are all vertical, the plant is 

 nowhere more than one cell thick. The marginal cells of the young seg- 

 ments divide more rapidly than the inner ones, and soon project beyond 

 the apical cell which thus comes to lie at the bottom of a cleft in the front 

 of the plant which in consequence becomes heart-shaped (E,,F). Sooner 

 or later the apical cell ceases to form regular segments and becomes indis- 

 tinguishable from the other cells. 



In the ostrich fern and lady fern the plants are dioecious. The male 

 plants (Fig. 66, 7) are very small, often barely visible to the naked eye, 

 and when growing thickly form dense, moss-like patches. They are 

 variable in form, some irregularly shaped, others simple rows of cells, 

 and some have the heart shape of the larger plants. 



The female plants (Fig. 66, F) are always comparatively 

 large and regularly heart-shaped, occasionally reaching a diam- 

 eter of nearly or quite one centimetre, so that they are easily 

 recognizable without microscopical examination. 



All the cells of the plant except the root hairs contain large and distinct 

 chloroplasts much like those in the leaves of the moss, and like them 

 usually to be found in process of division. 



The archegonia arise from cells of the lower surface, just behind the 

 notch in front (Fig. 66, F, ar.). Previous to their formation the cells at 

 this point divide by walls parallel to the surface of the plant, so as to form 

 several layers of cells, and from the lowest layer of cells the archegonia 

 arise. They resemble those of the liverworts but are shorter, and the 

 lower part is completely sunk within the tissues of the plant (Fig. 66, 

 6r, /). They arise as single surface cells, this first dividing into three by 

 walls parallel to the outer surface. The lower cell undergoes one or two 

 divisions, but undergoes no further change; the second cell (<7, o), 

 becomes the egg cell, and from it is cut off another cell (c), the canal cell 

 of the neck ; the uppermost of the three becomes the neck. There are 

 four rows of neck cells, the two forward ones being longer than the others, 

 so that the neck is bent backward. In the full-grown archegonium, there 

 are two canal cells, the lower one (//, 6) called the ventral canal cell, 

 being smaller than the other. 



