4O FIRST GROUP. THALLOPHYTES. 



roundish disks containing a large number of cells (Fig. 19), which are either connected 

 with one another with no intercellular spaces, or form a perforated disk. 



The coenobia in Pediastrum are propagated by the formation of a large number of 

 swarm-cells in each cell, which move about for some time within the mother-cell, 

 and coming to rest arrange themselves in a definite order, which varies according 

 to the species ; they then increase in size together and eventually multiply again in the 

 same way, as Fig. 19 shows." Certain smaller swarm-spores have also been observed, 

 which escape from the mother- cell, and, to judge by Hydrodictyon^ must certainly be 

 gametes. In Hydrodictyon utriculatum, which occurs here and there in wet ditches, 

 the full-grown plant, the coenobium, consists of a sac-like net several centimetres 

 long, formed of a large number of cylindrical cells with their ends so united 

 together as to form 4-6-angled meshes. In the ordinary mode of reproduction the 

 green contents of one of the cells of a net break up into 7000-20,000 swarm-cells ; 

 these exhibit the usual swarming motion inside the mother-cell, and coming to rest at 

 the end of half-an-hour arrange themselves in such a manner that when they elongate 

 they produce a net of the original form, which is set free by the dissolution of the wall 

 of the mother-cell, and in three or four weeks attains the size of the parent-plant. The 

 green contents of other cells of a mature net, on the contrary, break up into 

 30,000-100,000 smaller swarm-cells, which escape from the mother-cell at once and as 

 they escape conjugate in pairs, seldom more together. The zygospores move freely 

 about for a longer time ; on coming to rest they become spherical in form, secrete 

 a firm cell-wall, and can remain in a dry state for the space of some months, if not 

 exposed to the light. They begin slowly to grow again after several months' rest. 

 After they have reached a considerable size their contents break up into from two to 

 four large zoospores, which coming to rest after some minutes assume a peculiar 

 angular form, and increasing considerably in size put out horn-like processes. The 

 green parietal contents of each of these so-called polyhedra break up once more into 

 zoospores, which move about for from twenty to forty minutes inside a sac, which is 

 the swollen inner membrane of the polyhedron and protrudes from it. When they 

 come to rest these cells arrange themselves within the sac, which eventually disappears, 

 into a very small sac-like net composed of from 200-300 cells, which resembles in 

 other respects the ordinary nets and gradually grows to their size. In many polyhedra 

 smaller and more numerous zoospores are produced, which however unite in the same 

 way to form a net. 



b. Of the free-living Protococcaceae which do not form united colonies one species 

 may be mentioned, which lives in the cavities of the tissues of other plants and has 

 been recently and carefully investigated by Klebs *. Chlorochytrium Lemnae is 

 a unicellular Alga that is found in the intercellular spaces of Lemna trisulca. Each 

 individual, without previously multiplying by bipartition, produces zoospores which 

 escape from the tissue of the Lemna and are enclosed in a gelatinous envelope. These 

 zoospores are gametes and conjugate inside the gelatinous vesicle ; the zygozoospores^ which 

 have four cilia, escape into the water, and after swimming about there for a short 

 time settle on the epidermis of Lemna trisulca^ and always on the boundary line of 

 two epidermal cells. Here they come to rest, invest themselves with a membrane, 

 and put out a colourless process which thrusts apart the walls of the epidermal cells ; 

 eventually the protoplasm of the zygospore passes into the process. This proceeding 

 is repeated several times in the course of the period of vegetation. During the winter 

 the chlorochytrium-plants inside a Lemna become resting cells, which in the ensuing 

 spring again produce gametes. Many Protococcaceae besides Chlorochytrium live 

 within other plants and are well known ; Phyllobium dimorplium may serve as an 

 example of them. This Alga is found in the tissue of the vascular bundles of the 

 leaves of Ajuga reptans and other plants, but especially in Lysimachia Nummularia. 



1 Beitrage zur Kenntn. niederer Algenformen (Bot. Zeit. 1881, p. 249 ff.). 



