252 THALLOPHYTES. 



Myxomycetes a conjugation of zoogonidia does take place if the term is used 

 in its most extended sense ; for there is absolutely no reason why the coalescence 

 of the Myxoamoeba^i should not be regarded as a form of conjugation, and the 

 production of plasmodia as analogous to that of zygospores. What has hitherto 

 prevented botanists from recognising this analogy is merely the habit of life of 

 the Myxomycetes, especially the peculiarity of not forming cells, and the cir- 

 cumstance that conjugation takes place between thousands of zoogonidia. If the 

 coalescence of the myxoamoebae into a plasmodium is compared to the conjugation 

 of the zoogonidia of the Pandorineae, then the fructification which results from the 

 Plasmodium must be regarded as a large zygospore, the contents of which break up 

 into a number of spores, like the large resting-spore of Synchitrium. They have at 

 first, it is true, no power of motion, but this is quite a secondary consideration if 

 regard is had to the corresponding processes in the Pandorinese. 



With reference to the structure of the thallus, all the plants included in this 

 class may be termed unicellular, the thallus consisting of a single independent cell, 

 an eremoblast ; or, when it is multicellular, the separate cells are nevertheless essen- 

 tially similar, so that their union is scarcely necessary physiologically for the existence 

 of the whole. Such unions of equivalent cells, the product of a single mother- 

 cell, sometimes take place only after the individual cells have passed through 

 a kind of swarming phase ; when they come to rest, a number of cells unite 

 to form the so-called Ccenobium, and continue their development as a body with 

 definite form. This formation of coenobia, as it occurs in Hydrodiciyon and Pedi- 

 antrum, exhibits a certain analogy, not only with the process of conjugation generally, 

 but with the formation of plasmodia in the Myxomycetes in particular, except that 

 a Plasmodium, if regarded as a ccenobium consisting of masses of naked protoplasm, 

 no longer manifests any independence of the separate parts. 



The formation of a tissue in the ordinary sense of the term occurs only in a few 

 Algse, as Ulothrix and some Conjugatae, in so far that the divisions take place in 

 one direction only, and the cells thus produced remain more or less firmly united, 

 forming filaments in which all the cells are still perfectly equivalent, each one of them 

 representing therefore the entire plant. The predominant unicellular character is 

 manifested also in individual cells being capable of a high development, especially 

 in the Conjugatae, Diatomaceae, and Zygomycetes. This tendency is manifested 

 in the former in the contents of the cells and in the peculiar sculpture of the 

 cell-wall, in the Zygomycetes in the extremely complicated branching, while the 

 cell-contents remain simple. 



The greater number of forms belonging to the class, especially those which 

 contain chlorophyll, the Pandorineae, Hydrodictyeae, Desmidieae, and Diatomaceae, 

 exhibit in their thallus no distinction between base and apex; they form either 

 spherical or tabular coenobia or rows of cells, all of which multiply by division 

 without any distinction between base and apex; but in some Diatomaceae and at 

 all events in the germination of the zygospores of the Mesocarpeae and Zygnemeae 

 adhesion takes place to a substratum, and hence a kind of contrast arises between 



^ See Brefeld, Ueber Dictyostelium mucoroides in Abhandl. des Senkenb. Gesellsch., vol. VII. 

 1869, p. 20. 



