492 LECTURE XXIX. 



and this case is very common. The genus of non-cellular Algae Caulerpa, so often 

 referred to already, presents a clear example of this. As shown in Fig. 327, the 

 shoot-axis s v is, sharply segmented off from the roots w and leaves b. The shoot-axis 

 creeps horizontally, and its dorsi-ventrality comes out far less in its anatomical structure 

 than in its producing exclusively roots on the under side and exclusively leaves on the 

 upper side; and when lateral shoots originate from the shoot-axis (which rarely 

 happens however) these come forth alternately on the right and left flanks. It may 

 here also be mentioned in anticipation that the roots and leaves which spring from 

 the dorsi-ventral, and therefore plagiotropic shoot-axis, are, in their turn, orthotropic : 

 the roots grow vertically downwards and branch radially, the leaves grow vertically 

 upwards and branch bilaterally. Just as in this unusually clear case, the dorsi- 

 ventral character of the branching occurs in other Algae which undergo cell-divisions 



Wmm 



M 



FIG. 327. — Canlerpa 



as they grow, and even in many highly organised Cryptogams and Phanerogams. The 

 entire group of Rhizocarpeae behave like Caulerpa, and among the Phanerogams 

 Utriciilaria especially is to be mentioned, because in it the lateral shoots sprout from 

 the dorsal side and the leaves from the flanks of the dorsi-ventral axis ; among the 

 Ferns again the genus Lygodium is especially remarkable, because in it the leaves 

 arise in one series from the dorsal side of the dorsi-ventral creeping stem. 



In these and other cases it is the plumule which, gathering strength as the 

 main shoot, assumes the dorsi-ventral structure, so that the entire plant is dominated 

 by the dorsi-ventrality in its organisation. It happens much more frequently, how- 

 ever, particularly in Phanerogams, that the plumule possesses a radial structure 

 from the beginning, with its ofif-shoots on three or more sides, and grows 



