240 
THALLOPHYTES, 
large Fungi (Basidiomycetes), the sexual organs are still unknown ; although in 
the latter case the analogy of the Ascomycetes renders it very probable that the 
Fungi known as Hymenomycetes and Gasteromycetes are only the fructification 
which is the result of the union of sexual organs on the mycelium. The spores 
which are produced on these Fungi must t'^erefore be treated as true spores in 
our sense of the term, and their mode of formation as something quite different 
from that of the gonidia of the Mould-fungi. 
It is very common among Algae, and occurs also in some Fungi which grow 
in water or on a moist substratum, for the gonidia when they escape from the mother- 
cell to be naked, i.e. without any cell-wall, and motile; after their escape they 
have for some minutes or even hours the power of swimming about, at the same 
time rotating on their axis (swarming). The anterior end is hyaline, destitute of 
granules or colouring matter ; and in some Algge a minute red dot hes at one side 
behind the hyaline part ; the cause of the motion is the vibration of certain very fine 
threads, the Vibratile Cilia. Usually two of these cilia are attached to the hyaline 
anterior end, or one in, front, the other at the side ; but sometimes there is only 
one, while in others the hyaline anterior end is encircled by a dense circlet of 
numerous cilia ; or finally the entire surface of the zoogonidium is covered with 
short ciHa. During swarming a cell-wall of cellulose begins to be secreted; the 
zoogonidium then comes to rest, attaches itself to some solid body by its anterior 
end, the cilia disappear, and germination commences, the end which was posterior 
during swarming becoming the growing point and hence the anterior end of the 
young plant. It has already been mentioned that in some cases swarming cells 
conjugate, and these must then of course be regarded, not as gonidia, but as sexual 
organs which bear only a deceptive resemblance to zoogonidia; at any rate 
there are reasons for beUeving that the motile cells of some Algse which have 
hitherto been regarded simply as gonidia, are capable of conjugation and are 
therefore sexual organs. 
Motile cells of the kind now described may make their appearance at any stage 
in the course of development ; it is not uncommon, as we have seen, for the entire 
contents of an oospore or even of a carpospore (as in ColeochcEte) to be transformed 
into motile cells which can then germinate ; even in the so-called conidia of the 
Peronosporese the whole of the contents may break up into motile cells. In other 
cases again these bodies are produced in special branches of the thallus, and not 
unfrequently any vegetative cell of the thallus may allow its whole contents to 
escape in the form of motile cells. These motile cells have hitherto been all 
known as Swarm-spores or Zoospores ; but, according to the definition of the term 
Spore which we have now adopted, we must term the asexually produced motile 
cells Zoogonidia, and designate the receptacles in which they sometimes arise in 
large numbers, not zoosporangia, but Zoogonidia-receptacles. It is moreover 
obviously of secondary importance whether the gonidia simply become detached, as 
in most Fungi, or whether they take the form of motile cells. The difference is 
evidently dependent on the mode of fife of the plant ; the presence or absence of 
the power of swarming is not one of morphological, but only of physiological 
importance ; just as, in the seeds and fruits of Phanerogams, some have a power 
of transportation by means of a special floating-apparatus, while others simply fall 
