474 



LECTURE XXVin. 



and constitute branches of the shoot. The behaviour of a dichotomy of course 

 becomes more complicated when leaf-forming shoots are concerned, as in the 

 Lycopodiaceae, where in all the genera and species not only the shoots but also 



the roots branch in a dichotomous manner. 

 In contrast to the ordinary lateral, or as 

 it is also termed monopodial branching, 

 there is this characteristic feature, that the 

 formation of the two new growing-points 

 from the older one takes place not only 

 above the youngest leaf (simply because 

 it takes place in the apex itself) but it also 

 stands in no relation whatever to the pre- 

 ceding leaves. 



A long misunderstood but nevertheless 

 very simple case of dichotomy, and one 

 which is not essentially diiferent from 

 those hitherto mentioned, is found in those 

 Liverworts which possess broad band-like 

 creeping shoots, especially the Marchantiese 

 and leafless Jungermanniese. If we put 

 aside the sequence of cell-divisions, which 

 have been studied in every detail, as well as the presence- or absence of an apical 

 cell, here also the dichotomy consists in that the group of embryonic tissue at the 

 base of the deep depression previously described becomes slightly extended in 

 breadth, as shown in Fig. 313; the middle portion of this embryonic tissue then 



FIG. 312. — Dichotomously branched shoot of 

 Dictyota dichalama, a marine Alga. 



Fig. 313.— Flat dichotomously branched shoot Hi Mttzgerietfurcata 

 (X about 15 ) ; m m mid -rib of several layers, dichotomising at^^ ; s s grow- 

 ing-points, with projecting wing-like outgrowths./^ /"". 



grows vigorously, and becomes transformed into permanent tissue, which now 

 projects in the form of a wedge arched in front, while right and left of it the 

 rest of the embryonic tissue remains as such and now constitutes two growing-points 

 developed from one. 



