THALLOPHYTA. 639 



fact that Lemna trisulca lives quite happily and can flower when infested with 

 Chlorochytrium, and that the germ tubes o£ Phyllobium dimorphum usually enter 

 dead leaves of the Moneywort. Another form which always enters the living leaves 

 of a river-weed, continues its course of development whether the leaves die or re- 

 main alive. It is not, however, difficult to imagine how a form like Phyllohiwm, 

 living as it does in the vascular bundles of its host, might acquire a parasitic habit 

 by tapping the food supplies. As a matter of fact certain confervoid Algse are 

 known whose presence results in the death of the leaves they inhabit, though pro- 

 bably not by direct appropriation of the food of the host. 



Resembling the Endosphcerece in possessing motionless cells which form zoospores 

 but do not undergo vegetative divisions, are certain common fresh- water forms of 

 which Characium and SciadiuTn may be mentioned. A plant of Sciadium ori- 

 ginally consists of a single cylindrical cell whose contents breaks up into zoospores. 

 These zoospores have acquired the peculiar habit of settling on the rim of the 

 mother-cell, instead of seeking out fresh spots for their development. Each zoospore 

 produces a single cell like the mother, so that a whorl of cells of the new genera- 

 tion is formed on the top of the original cell. This process may be repeated for 

 two or three generations, after which the zoospores will settle on some other object 

 and start fresh " plants ". 



The HydrodictyacecB are a group of Algse which form immotile colonies. The 

 cells of these colonies resemble the single cells of the forms we have just been con- 

 sidering in producing zoospores or gametes, but undergoing no vegetative divisions. 

 The colony is formed by the joining together in a definite way of the group of 

 zoospores formed in a single cell of the mother-colony. Each of these zoospores 

 then develops into an adult vegetative cell. 



The recently discovered genus Euastropsis (so called from its likeness to the 

 Desmid Euastrum) is the simplest type of the family. It consists of two mitre- 

 shaped cells joined to one another by their bases. Each cell contains a parietal 

 chromatophore with a single pyrenoid, and a single nucleus. The contents breaks 

 up by successive divisions into 2-32 zoospores, which escape from the cell surrounded 

 by a general membrane. After oscillating for about a quarter of an hour, the zoo- 

 spores become attached in pairs by their anterior ends. Each pair then takes on 

 the characters of the two-celled colony. 



Pediastrum (fig. 370 ^) consists of a disc of cells, of which the marginal ones are 

 often drawn out into lobes or processes. The chromatophore is parietal with a 

 single pyrenoid; there are numerous nuclei. The formation of zoospores is like that 

 of Euastropsis, but their movement is more lively, and eventually all the zoospores 

 formed in a single cell join together to form a new Pediastrurn-colonY (figs. 370 " 

 and 370 *). Gametes are formed in the same way as the zoospores, but are smaller 

 and more numerous. They escape from the investing membrane, swim freely in 

 the water, and fuse in pairs to form zygotes. From these zygotes new Pediastrum- 

 colonies are produced indirectly, probably by a method like that obtaining in 

 Hydrodictyon. 



