THE ALG^E 173 



interval of rest. The zygospore can now survive either a 

 period of drought or the cold of winter, as the case may be. 

 On germination the zygospore, unlike that of Ulothrix and 

 its allies, gives rise directly to a new plant. The outer 

 wall is burst, the protoplasm grows out, clothed only by 

 the inner cellulose wall, the bright green colour of the 

 chlorophyll reappears, and starch is once more formed. 

 On first germinating, the young plant shows a distinction 

 between apex and base, for it remains for a time attached 

 by one end, which is pointed and colourless. This dis- 

 tinction is soon lost, and the filament floats freely in the 

 water. In a few cases, where conjugation fails to take 

 place, the single cells have been observed to form resting- 

 spores asexually, and in one or two species this is the rule. 

 The life-history of Spirogyra, which represents a con- 

 siderable order of fresh -water Algae, is, as we see, 

 extremely simple ; there is not the slightest indication 

 of any alternation of generations. All individuals alike 

 are capable of conjugation, and, are therefore sexual. 

 The name Conjugate is applied to this order, for it was 

 among them that the union of similar cells to form a 

 zygospore was first observed. In this group it is always 

 the ordinary vegetative cells which unite, so the pro- 

 cess is essentially different from the union of actively 

 moving spores such as takes place in Ulothrix. The 

 Conjugatse appear to be an isolated group, showing no 

 near relation to any of the higher forms, whereas from 

 the motile conjugation of Ulothrix we can trace a series 

 of steps leading on to the well-marked fertilisation of 

 Algae like (Edogonium. In 8pirogyra t however, we have 

 really the type of all sexual reproduction, which consists 

 essentially in the fusion of two distinct cells, accompanied 

 by the union of their nuclei. The structural differences 



