THALLOPHYTA 31 



Macrocystis, the Laminarias, show a secondary growth in 

 thickness of the stalk, the merismatie 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 Draccena, though the cells formed are different, 

 never becommg lignified nor truly vascular. There is formed 

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

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

 plants of the same group certain secretory structures, the mucus 

 or gu77i loassages, are differentiated. In the masses of tissue 

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

 such as Agaricus, the structure is composed of filamentous cells 

 or hyphffi 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 thej- 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 smaU structure not detached from the gametopln'te, and 

 sometimes consisting only of a single ceU. 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 ThaUophytes, when sporoph>'te 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 jjo^en^irtZ one. In many cases a succession of 

 potential gametophytes occiu-s, 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 hojnologous alternation, to 

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

 thetic. 



A potential gametophyte must not be confiTsed with a sporo- 

 phyte, though both bear only asexual reproductive cells. The 



