IV. 



THE ANTIIOCEROTES 125 



water-supply through the drying up of the thalhis, the sporo- 

 phytc hnally dies. 



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 1)y a 

 greater or smaller number of intervening cells. It is some- 

 what difficult to determine positively whether one or move 

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

 initial cells are seen to occupy the bottom of a shallow depres- 

 sion (Fig. 65, C). In the case figured, x probably is the single 

 apical cell, and it seems likely that this is usually the case, al- 

 though 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. 66) shows that the 

 apical cell is much larger than appears from the horizontal 

 section. On comparing the two sections it is evident that its 

 form is the same as in the Marchantiace?e 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 

 i:he 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 Mar- 

 chantiaceae. From the outer cells are developed the special 

 superficial organs both on the ventral and dorsal sides. From 

 the former arise the colourless delicate rhizoids and peculiar 

 stoma-like organs, the mucilage clefts, first described by 

 Janczewski ( i ) , W' ho also pointed out the true nature of the 

 Nostoc colonies found w^ithin the thallus. These mucilage 

 clefts, especially in their earlier stages, resemble closely the 

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

 aration 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 neighboring 

 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 wath the real stomata 



