l^Z 



THALLOPHYTES. 



transitional forms. The one extreme is formed by C. divergens, which, as it developes 

 from the spore, produces first of all creeping irregularly branched articulated threads, 

 from which spring ascending articulated branches which are also irregularly branched ; 

 the whole thallus not assuming any definite form. In C. pul-vinata, on the contrary, the 

 thallus forms a-hemispherical cushion ; the cellular filaments which are the result of germi- 

 nation branch somewhat irregularly in one plane, but form something of a disc ; from 

 them rise up ascending articulated branches, which again branch and form the cushion. 

 In the following species no ascending branches are formed, but those which cling to 

 their support form a more or less regular disc. In C. irregularis this takes place by irre- 

 gular ramifications which lie in one plane gradually filling up all the interstices, till an 

 almost uninterrupted layer of cells is obtained. In C. soluta (Fig. 164), on the other 

 hary^, a dichotomous ramification commences in the two first daughter-cells of the ger- 

 minating spore, with corresponding cell-division of such a nature that even at a very 

 early period a close disc of radial forked branches is formed, which either lie loosely or 

 are closely crowded side by side. While in the species already -named the branches arise 

 laterally from cells, but never from the terminal cell of a branch, in C. soluta we have the 

 first instance of dichotomy in the regular disc-shaped centrifugal growth ; a process 

 which attains the highest development in C scutata. In this species the first cells which 

 result from germination remain from the first united laterally and do not form isolated 

 branches; the circular disc, when once formed, continues to grow by increase of its 

 circumference, the marginal cells dividing by radial and tangential walls. This mode 

 of growth may be referred fundamentally to the law that the first twigs united laterally 

 grow with equal rapidity in a radial direction, and then become divided by septa 

 (in this case tangential) ; while the broadening of the terminal cell of each radial row 

 corresponds with the succeeding radial division of a dichotomy. The law which prevails 

 in the species already mentioned, that only t-he terminal cell of a branch is divided by 

 septa, is exemplified in C. scutata by the marginal cells only of the disc being divided 

 by tangential walls. 



The Reproduction of the Coleochaetae is brought about by asexual swarm-spores and 

 by resting oospores produced sexually. The oospores do not at once produce new 

 plants, but several swarm-spores ; and the following alternation of generations takes 

 place : — The first swarm-spores, which arise in the early part of the year at the com- 

 mencement of vegetation from the cells of the oogonia of the previous year, produce 

 only asexual plants, or, in other words, only such as can form swarm-spores. Only 

 after a series of asexual generations varying in length does a sexual generation arise, 

 which may be either monoecious or dioecious according to the species. Fertilisation 

 produces one oospore in the oogonia which clothe themselves with a peculiar layer of 

 cortical cells ; and this oospore itself again developes into a parenchymatous reproductive 

 body, from the cells of which the first swarm-spores proceed in the next period of 

 vegetation (Pringsheim). The swarm-spores (Fig. 165, D) may arise in all the vegetative 

 cells of the Coleochaetae ; in C. puMnata especially from the terminal cells of the 

 branches; they are always formed from the entire contents of the mother-cell, and 

 escape through a round hole in its cell-wall. 



The oogonium is always the terminal cell of a branch, and hence in C. scutata 

 the terminal-cell of a radial row (Nageli). The peculiar mode of its development is 

 subject, according to the growth of the plant, to some, though subordinate, modifi- 

 cations. One species, C. puhnnata (Fig. 165), may first of all be examined somewhat 

 more closely. The terminal cell of a branch swells up and at the same time elongates 

 into a narrow bag (Fig. 165, A, og, to the left), which then opens {og" , to the right) and 

 exudes a colourless mucilage. The protoplasm of the swollen part which contains 

 chlorophyll forms the oosphere in which a nucleus is visible. The antheridia are formed 

 at the same time in adjoining cells, two or three protuberances {A, an) growing out, which 

 become separated by septa ; each of the cells thus formed, which have somewhat the 

 shape of a flask, is an antheridium ; its entire contents form a spermatozoid {%) of oval 



