372 BOTANY OF THE LIVING PLANT . 



to determine by altering the conditions what form of propagative organ shall 

 be produced. This has been shown with particular clearness in the case 

 of Vaucheria. If the plant be kept first in a flowing stream of water, as it 

 may be in a glass tube, it simply grows vegetatively. If it be then transferred 

 to still water, zoospores are produced. This also follows on flooding, in the 

 case of terrestrial forms. If it be desired to produce sexual organs, the plant 

 should be well nourished, for instance by exposure to good light, or cultivation 

 in a weak sugar solution. A rise of temperature also encourages their pro- 

 duction. Speaking generally, light produces sexual organs, shade zoospores. 

 A third type of propagative organ is formed in some species (V. geminata). 

 Under dry conditions the filament, dividing up into short lengths, forms 

 aplanospores with thick walls. These can stand drying up. All of these 

 propagative organs are biologically suitable to the spread and survival of 

 the plant in its native habitat. When flooded in cool weather it forms 

 zoospores. When there is risk of drying up in summer after exposure to 

 light and heat it forms zygotes or aplanospores, which can tide over the 

 period of drought ; but on germination, whether of the zoospores, zygotes, 

 or aplanospores, either zoospores or sexual organs may be formed first on 

 the young plant, according to the conditions. 



In Vaucheria it is probable that reduction takes place at the first nuclear 

 division in the germinating oospore ; and hence the vegetative plant is 

 typically haploid. But in Acetabularia and related genera the plant is diploid, 

 with reduction occurring at the formation of the gametes. 



CONJUGALES. 



A considerable series of common Green Algae belong to the Con- 

 jugates, a group which stands aloof from the Chlorophyceae in the 

 more restricted sense. One large family of these is the Zygnemaceae, 

 filamentous Algae native in still water : the most familiar example 

 being Spirogyra. Another family, the Desmidiaceae, are mostly 

 unicellular, and very beautiful. They are found in quantities in peaty 

 pools. The two families are grouped together because of the structure 

 of their uninucleate cells, which contain complicated chromato- 

 phores ; and because both show conjugation of non-motile gametes. 



The well-known genus Spirogyra includes numerous species, of 

 which the filaments float commonly unattached in still fresh water, 

 and with no distinction of apex and base. They are slimy to the 

 touch, owing to their outer wall being mucilaginous. Each filament 

 is partitioned by transverse septa into cells, each of which may be 

 detached from its neighbours by shock, when its convex ends demon- 

 strate its internal turgor. Growing on and dividing, each may form 

 a new filament. Each cell is practically an individual (Fig. 277, cell h). 

 It is cylindrical, the proportion of length to breadth varying in 

 different species. Within the external wall is a layer of colourless 

 cytoplasm surrounding a central vacuole, in the middle of which the 



