THALLOPHYTES. I I 



of the development by sexual organs and formation of true fructification being attained 

 only under unusually favourable circumstances. In other Fungi, as has been already 

 stated, there is usually no formation of fructification. 



Naked, that is, membraneless brood-cells which have the power of free move- 

 ment, occur frequently among the Algae, and also in some Fungi that live in water or 

 on moist substances. They swim about in the water for some minutes or even hours 

 after they are set free from the parent plant, and combine a rotation on their own 

 axis with a forward movement. The anterior extremity is hyaline, and free from 

 granules and colouring matter, and in many Algae a small red corpuscle lies behind 

 the hyaline portion ; the movement is due to the vibrations of very delicate threads 

 (cilia). There are usually two such cilia on the anterior extremity, or near it on the 

 side ; sometimes there is only one ; or the hyaline anterior extremity is surrounded 

 by a circle of numerous cilia, or finally the whole surface of the swarm-cell is 

 beset with short cilia. A wall of cellulose begins to be secreted during the time of 

 swarming; then the swarm-cell, coming to rest, attaches itself by the anterior 

 extremity to some body, the cilia disappear and germination begins, the posterior 

 end during the period of movement becoming the free growing point, and con- 

 sequently the anterior extremity of the young plant. It has been already stated that 

 conjugation takes place between swarming cells; such cells must obviously not be 

 regarded as brood-cells, but as sexual cells (gametes), which bear a deceptive 

 resemblance to motile brood-cells ; there is moreover reason for thinking that the 

 swarm-cells of many Algae, which have been hitherto taken for gonidia, are capable 

 of conjugation and are therefore gametes. Motile cells of the kind described may 

 appear at very different stages in the course of the development of the plant; it not 

 unfrequently happens that the cell-contents of an oospore, in Coleochaete for example, 

 breakup into swarm-cells, which then proceed to germinate; even brood-cells may 

 convert their contents into swarm-cells, as happens in the so-called gonidia of the 

 Peronosporeae ; in a different class of cases swarm-cells are formed in special 

 branches of the thallus, and any ordinary vegetative cell of the thallus may not 

 unfrequently discharge its contents in the form of swarm-cells. Hitherto all such 

 cells have been called swarm-spores or zoospores ; it would be convenient and in ac- 

 cordance with \vhat has been said on page 10, upon the idea of a spore, to employ only 

 the terms swarm-cells or zoogonidia, and to call the receptacles, which sometimes 

 contain a vast quantity of swarm-cells, not zoosporangia but zoogonidia-receptades. 

 It is obviously a matter of minor importance, whether the brood-cells simply fall off 

 from the parent plant, as is the case in most Fungi, where they are then usually 

 termed gonidia, and also in many Algae, or whether they appear as swarm-cells ; this 

 evidently depends on the mode of life of the plants; the free movement or its 

 absence is physiologically not morphologically important ; just as in the seeds and 

 fruits of Phanerogams, some have a special apparatus for flying and are capable of 

 movement, others simply fall from the plant to the ground. Moreover we find in 

 the genus Vancheria all stages of transition from free-moving swarm-cells to gonidia 

 that simply drop from the plant ; and still more striking is the state of the case in the 

 Fungi known as the Peronosporeae, where the forms which live in water or on a 

 moist substratum produce motile gonidia, those that are parasitic on land-plants have 

 non-motile gonidia. 



