22 PART I. MORPHOLOGY. [ 9, 10 



the intercalary growth of their common base (Fig. 12 A, r), and 

 come to be merely lappets on the rim of a tube (Fig. 12 }. 

 This explanation applies also to perfoliate and connate leaves (see 

 Fig. 20). 



The union brought about in either of these ways may affect 

 members developed at the same level, or members developed at 

 different levels ; in the former case the term cohesion is used ; 

 in the latter, the term adhesion. Examples of the former are 

 afforded by gamopetalous corollas, syncarpous ovaries, ta; and 

 of the latter by epipetalous stamens, by leaves adhering to the 

 shoots borne in their axils, as in the Lime, etc. 



9. The Thallus. The thallus offers considerable variety 

 of form. It may be spherical ; or filamentous, branched or un- 

 branched ; or a flattened expansion, branched or unbranched ; or 

 a massive tuberous body. It commonly bears hairs. The sym- 

 metry of the thallus is multilateral, isobilateral, or dorsiventral. 

 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, Coleochsete) with similar sur- 

 faces ; dorsiventral symmetry, when the thallus is flattened, with 

 dissimilar dorsal and ventral surfaces (e.g. most Hepaticse, and 

 Fern-pro thallia). 



The branching of the thallus takes place in accordance with the 

 general laws laid down in 7 ; the flattened thallus frequently 

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

 axis and the branches maybe 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 reproductive 

 organs is in some cases confined to certain branches, and these then 

 differ in form from the ordinary vegetative branches (e.g. some 

 Hepaticse). 



10. The Shoot. The shoot may be either thalloid (see p. 3) 

 or leafy. The morphology of the thalloid shoot hardly requires 

 special consideration : it is much the same as that of the thallus. 



The general form of the leafy shoot varies widely. Even on one 

 and the same plant there may be different forms of leafy shoots. 

 the differences being due either to peculiarities in the conditions 

 of development, or of function. Marked differences exist, for 

 instance, between submerged, or subterranean, and aerial shoots ; 

 also between vegetative shoots and those bearing the repro- 

 ductive organs. 



