268 
THALLOPHYTES. 
Antheridm, 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 Saprolegniese. With the exception 
of the extremely simple Spkceroplea, 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 ferdlisation 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 Zygosporese, 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 Sphcsroplea, 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 formadon 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, Sphceroplea, 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. 
