THALLOPHYTA 31 



Macrooystis, the Lamvnarias, show a, secondary growth in 

 thickness of the stalk, the merismatic layer, being sometimes the 

 epidermal one, and sometimes one specially differentiated near 

 the periphery. In the latter case the growth in thickness re- 

 sembles that of Dracmna, though the cells formed are different, 

 never becoming lignified nor truly vascular. There is formed 

 in some a kind of axial strand of elongated cells, which can 

 sometimes be traced upwards into the leaves. In some of the 

 plants of the same group certain secretory structures, the mucus 

 or gum passages, are differentiated. In the masses of tissue 

 constituting the so-called fructifications of the more bulky Fungi, 

 such as Agaricus, the structure is composed of filamentous cells 

 or hyphse arranged side by side and cohering together. 



The differentiation of tissue leads also to the localisation of 

 the regions of growth. The growing points may be convex 

 or depressed, when they are apical ; or they may be intercalary. 



The form in which the plant exists is always the gameto- 

 phyte. The sporophyte is either not produced at all, or it forms 

 only a small structure not detached from the gametophyte, and 

 sometimes consisting only of a single cell. In only a few cases 

 is it a separate structure. There is thus not that regular 

 alternation of generations which we have seen to exist in all the 

 forms above the Thallophytes, when sporophyte regularly gives 

 rise to gametophyte and is again reproduced from the latter. This 

 obtains, however, in some cases, particularly in the higher Algae. 



The gametophyte in very many cases produces asexual as 

 well as sexual reproductive cells. These are in structure and mode 

 of behaviour almost exactly like the spores of the sporophyte. 

 To distinguish them from the latter they are often called gonidia. 

 In some cases both gonidia and gametes are borne upon the 

 same gametophyte ; - in others the plant frequently produces 

 gonidia alone. A gametophyte in which no sexual cells arise is 

 spoken of as a potential one. In many cases a succession of 

 potential gametophytes occurs, each arising from one of the 

 gonidia of its parent, which succession is sooner or later 

 interrupted by one of the series producing sexual cells. Thus an 

 irregular alternation of generations arises, not between gameto- 

 phyte and sporophyte, but between actual and potential gameto- 

 phytes. This is spoken of as homologous alternation, to 

 distinguish it from the other kind, which is known as anti- 

 thetic. 



A potential gametophyte must not be confused with a sporo- 

 phyte, though both bear only asexual reproductive cells. The 



