CHAP. V THE ANTHOCEROTEJL > 119 



In order to study the apical growth satisfactorily, young 

 plants that show no signs of the sporogonia should be selected. 

 In A. fiisiforuiis such a plant will show the margin of the 

 thallus occupied by numerous growing points separated by a 

 greater or smaller number of intervening cells. It is somewhat 

 difficult to determine positively whether one or more apical cells 

 are present. In sections parallel to the surface the initial cells 

 are seen to occupy the bottom of a shallow depression (Fig. 

 56, C). In the case figured, ;i:r probably is the single apical cell, 

 and it seems likely that this is usually the case, although 

 Leitgeb ^ was inclined to think that there were several marginal 

 cells of equal rank. The outer wall of the cells shows a very 

 marked cuticle. A vertical section passing through one of the 

 growing points (Fig. 57) shows that the apical cell is much 

 larger than appears from the transverse section. On comparing 

 the two sections it is evident that its form is the same as in the 

 Marchantiaceae or Pallavicinia. Two sets of lateral segments, 

 and two sets of inner ones, alternately ventral and dorsal, are cut 

 off, and the further divisions of these show great regularity, 

 this being especially the case in the dorsal and ventral segments. 

 Each of these first divides into an inner and an outer cell. The 

 former divides repeatedly and in both segments forms the 

 central part of the thallus. It is these cells that, according to 

 Leitgeb,^ later show thickenings upon their walls somewhat like 

 those met with in many Marchantiaceae. From the outer cells 

 are developed the special superficial organs both on the ventral 

 and dorsal sides. From the former arise the colourless delicate 

 root-hairs and peculiar stoma-like organs, the mucilage clefts, 

 first described by Janczewski,^ who also pointed out the true 

 nature of the Nostoc colonies found within the thallus. These 

 mucilage clefts, especially in their earlier stages, resemble closely 

 the stomata of the higher plants. They arise by the partial 

 separation of two adjacent surface cells close to the growing 

 point, and often at least the two cells bounding the cleft are 

 sister cells. However, the same division of the neighbouring 

 cells frequently occurs without the formation of a cleft, and there 

 is nothing to distinguish the two cells bounding the cleft from 

 the adjacent ones, and a homology with the real stomata on the 

 sporogonia is not to be assumed. The mucilage slit becomes 



^ Leitgeb (7), vol. v. p. 13. ^ Leitgeb, I.e. 



■* Janczewski (i). 



