38 PART I. THE MORPHOLOGY OF PLANTS. [ 9, 10. 



an exception, the vegetative body, apart from the reproductive 

 organs of the sporophyte, is a thallus (e.g. Wolffia arrhiza). 



The thallus offers considerable variety of form. It may be 

 spherical; or filamentous, branched or unbranched; or a flattened 

 expansion, branched or unbranched ; or a massive tuberous body. 

 It commonly bears hairs. The symmetry of the thallus is 

 multilateral, isobilateral, or dorsi ventral. Complete multilateral 

 symmetry is exhibited when the thallus is spherical (e.g. Volvox, 

 Fig. 1) ; isobilateral symmetry when the thallus is flattened 

 (e.g. Desmids, Coleochaete) with similar surfaces; dorsiventral 

 symmetry, when the thallus is flattened, with dissimilar dorsal 

 and ventral surfaces (e.g. most Hepaticae, and fern-prothallia). 



The branching of the thallus takes place in accordance with the 

 general laws laid down on p. 32 ; the flattened thallus frequently 

 branches dichotomously (e.g. some thalloid Hepaticae). The main 

 axis and the branches may be either limited or unlimited in 

 growth. 



The branches of the thallus may be modified in form in connexion 

 with some special function. Thus, the development of repro- 

 ductive organs is in some cases confined to certain branches, and 

 these then differ in form from the ordinary vegetative branches 

 (e.g. some Hepaticae). 



9. The Thalloid Shoot. The body is differentiated into 

 a root and a thalloid shoot ; in the gametophyte of some Algae 

 (some Siphonoideae, Confervoidese, Phseophyceae, and Florideffi), 

 and of some Vascular Cryptogams (Equisetum, Osmunda, Lyco- 

 podinm) ; in the sporophyte of some Algse (e.g. Dictyotaceae) and 

 of the Bryophy ta (except Riccia) . 



The vegetative body of the sporophyte has a thalloid shoot in some Phane- 

 rogams (e.y. Lemna). 



The morphology of the thalloid shoot is very much the same as 

 that of the thallus. In some cases, however (e.g. Laminaria, see 

 Fig. 10, p. 22), it is differentiated into a basal cylindrical stalk- 

 like portion, and a terminal flattened thalloid expansion. The 

 branching is, not uncommonly, dichotomous (e.g. Lictijota 

 dichotoma, Fig. 8, p. 19). 



10. The Leafy Shoot. The shoot is differentiated into 

 stem and leaf in some Algae (e.g. Caulerpa, Bryopsis, Cladoste- 

 phus, Sargassum, the Characeas, and the gametophyte of some 

 Floridese) ; the adult gametophytic shoots of some Hepaticro 



