240 THALLOPHYTES. 



large Fungi (Basidiomycetes), the sexual organs arc 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 therefore 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 hyahne, destitute of 

 granules or colouring matter ; and in some Algse a minute red dot lies at one side 

 behind the hyaline part j the cause of the motion is the vibration of certain very fine 

 threads, the Vibraiile 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 *^ense circlet of 

 numerous cilia ; or finally the entire surface of the zoogonidium is covered with 

 short cilia. 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 believing that the motile cells of some Algae 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 ColeochcBie) to be transformed 

 into motile cells which can then germinate ; even in the so-called conidia of the 

 Peronosporeae 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 life 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 



