268 THALLOPHYTES. 



Anlheridia, which penetrate into the oogonia and fertilise the oospheres; or a 

 kind of conjugation takes place between the antheridium and oogonium, as 

 Pringsheim has shown to be the case in the Saprolegnieae. With the exception 

 of the extremely simple Sphcei-oplea, the antheridial cell is always smaller than 

 the oogonium and different in form ; the motile antherozoids, which are pro- 

 duced either by simple contraction of the contents, or more commonly by division 

 into a great number of portions, are always much smaller than the oosphere, 

 and generally many hundred or thousand times. This great difference in size is 

 the essential difference between this mode of sexual union and conjugation, be- 

 sides the fact that the immotile oosphere passively awaits fertilisation by the 

 antherozoids which swim around it. After fertilisation the Oospore which results 

 from the oosphere behaves precisely as a zygospore; it becomes invested by a 

 firm cell-wall, and (except in Fucaceae) must undergo a period of rest before 

 germinating. Germination is in most cases indirect ; i. e. the contents of the 

 oospore do not at once develope into a new plant, but divide into a smaller 

 or larger number of cells which escape in the form of naked zoospores, each of 

 which grows into a new plant. In such cases therefore the oospore, as we have 

 repeatedly seen in the Zygosporeae, may be regarded as a very simple sporocarp 

 or as a second generation. The oosphere is the analogue of the oosphere in 

 the archegonium of Mosses; the ripe oospore, with its contents which break up 

 into zoospores, is the very simple equivalent of the moss-capsule, as has been 

 pointed out by Pringsheim, to whose researches we owe almost all that is here 

 described. But cases occur, as in the Zygosppreae, in which the oospore germi- 

 nates directly, as in the Fucacese, and there is no period of rest. 



The vegetative body of the Oosporeae may consist of undifferentiated cells, as 

 in Sphceroplea, in this respect resembling the Conjugatae, the filiform thallus present- 

 ing no distinction of base and apex. In one large group (Coeloblastae) the thallus 

 consists, until the formation of fructification, of a single tubular cell, which often 

 branches copiously, as in the Zygomycetes; in a further grade of development 

 the thallus consists of branched and segmented filaments composed of cells of 

 different kinds, and the plant, which is fixed to a substratum, manifests a well-- 

 marked contrast between base and apex. Finally we find the Fucaceae, in which 

 the thallus is very massive, and forms an actual tissue in which differentiation 

 may be recognised into epidermal layers and fundamental tissue. 



There is no non-sexual multiplication by gonidia, either in the simplest form 

 belonging to the class, Sphceropka, or in the most highly developed, the Fucaceae. 

 The thallus of the other intermediate forms, on the contrary, produces abundance 

 of gonidia, by means of which propagation may take place through many genera- 

 tions, in the same manner as Marchantia is propagated by bulbils. The gonidia 

 are produced either singly or in numbers as endogonidia in the interior of cells, 

 escaping in the form of zoogonidia, or as stylogonidia by abstriction at the extremity 

 of special branches ; they are then immotile, as in the Peronosporeae, though their 

 contents may become transformed into motile zoogonidia. 



In a- systematic classification of the Oosporeae, the forms which do not contain 

 chlorophyll may be arranged as a special section of those that do, the genetic 

 relationship being here unquestionable. 



