39 BOTANY OF THE LIVING PLANT 



filament, which is then shut off from the oogonium by a wall. Mean- 

 while a beak has formed at the distal end of the oogonium, which then 

 opens by swelling of the wall, a portion of the colourless contents 

 being emitted. The uninucleate egg then lies open for fertilisation by 

 the small motile spermatozoids (Fig. 335, v.). It appears that self- 

 fertilisation from an adjoining antheridium is the rule. The fusion of 

 the two nuclei has been observed to form the nucleus of the zygotc. 

 Then follows storage of further oil, a change of colour of the contents, 

 and the formation of a thick wall. In this state, as an oospore, a period 

 of rest follows. Germination takes place by rupture of the thick 

 wall, and the direct formation of a new filament from the contents. 



In such organisms as these it has been found that there is no obligatory 

 succession of events in the life-history. It lies in the hand of the experimenter 

 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 vegetativety. 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 

 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 

 either zoospores or sexual organs may be formed according to the conditions. 



CONJUGATAE. 



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

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

 more restricted sense. One large family of these are 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, with complicated chromatophores ; 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, 



