MARSILIA VESTITA. 



THE ROOT. 



As in the other members, the apical cell of the root is 

 at first external, and it is not until at least one complete 

 series of segments has been cut off from it that the first 

 segment of the root-cap is cut off; so that we may fairly 

 say that the first root is of exogenous origin, and in no 

 sense adventitious. It does not differ in an}^ particular 

 in its method of division from that of Pilularia which the 

 writer* has described at length, and will not therefore be 

 further treated here. The root occupies by far the 

 greater part of the hypobasal half of the embryo, and 

 the foot is comparatively little developed, and does not 

 show any definite succession in the cell division. While 

 m.orphologicallv, perhaps, only the lower hypobasal quad- 

 rant is to be regarded as the foot, physiologically the 

 whole lower surface of the embr3'o acts as such, and 

 therefore in this sense, the foot must be said to owe its 

 origin in part to the stem as well as to the root quadrants. 



At first there is very little increase in the size of the 

 embryo, the divisions being accompanied by very little 

 growth (compare figures 15, 16 and 17). When growth 

 does begin, however, it is very rapid, and within a few 

 days the embryo breaks through the overlying prothallium 

 cells, and the first cotyledon is then visible to the naked 

 eye as a fine green point. The cotyledon is the first part 

 of the embryo to break through the prothallium, but is 

 quickly followed by the root, which bends down and soon 

 penetrates the mud and fastens the 3^oung plant to the 

 ground. During this rapid growth the contents of the 

 large spore cavity are rapidly consumed, and serve to 

 support the young embryo until it can lead an independ- 

 ent existence. 



*1. c. p. 253. 



