Spirogyreae 353 



were inimical to it. Thus, if a Spirogyra be transferred from a dilute nutrient 

 solution to rain-water or tap-water and placed in bright light conjugation is 

 induced. Conjugation is also said to be induced by placing the filaments in 

 a 2 4 per cent, solution of cane sugar and exposing to sunlight. The student 

 must not, however, hope to be uniformly successful in attempts to induce con- 

 jugation, since many such attempts fail hopelessly, probably because of the lack 

 of the requisite combination of operative factors. Klebs also found that the 

 formation of parthenospores could be induced by placing conjugating filaments 

 in a 60 per cent, sugar solution. In these cases of apogamy the parthenospores 

 are usually of the same form as the zygospores and they germinate in the 

 same way. Parthenospores are not infrequently produced under natural con- 

 ditions and have been recorded by numerous observers. 



The case of Spirogyra mirabilis (Hass.) Petit is peculiar and may be the 

 result of degeneration, the formation of spores being apparently analogous to 

 that which occurs in Gonatonema. In spore-formation the cells become 

 swollen in the median part and the protoplast divides into two parts which 

 subsequently fuse together (Petit, '80). The cytology of this process would 

 be of great interest, but it still requires investigation. 



In the genus Sirogonium, the sterile threads of which differ little from 

 Spirogyra except in the somewhat irregular course of the chloroplasts, the 

 conjugation is comparable with that of Temnogametum. Certain cells of the 

 filaments become bent into a knee-like form, after which they attach them- 

 selves to similar bent cells of other filaments, the attachment being rendered 

 firm by a ring of tough mucilage. The protoplasts of the attached cells now 

 divide and partition-walls appear, cutting off either one or two smaller cells 

 from the remaining cell, which is the gametangium. Dissolution of the wall 

 occurs at the point of attachment of the gametangia and the zygospore is 

 formed in the female gametangium. 



The genera of the Spirogyrese are : Spirogyra Link, 1820, and Sirogonium Kiitzing, 1843 

 [ = Choaspis S. F. Gray, 182 1 1 ]. 



Species of Spirogyra, which as a genus is one of the commonest laboratory types 

 studied among the Algse, occur mostly in still waters and very largely in ponds and ditches, 

 where they often form floating masses buoyed up by numerous bubbles of oxygen. These 

 flocculent masses gradually change colour from green to yellow, mostly owing to the death 

 of a large proportion of the filaments as a result of exposure to light of too great an 

 intensity, although sometimes owing to almost universal conjugation. Some species 

 habitually occur in very slow streams and rivers. Sirogonium sticticum Kiitz. generally 

 occurs attached to stones over which the water is quickly running. It is not infrequent 

 in limestone districts and is noteworthy for the comparatively -small amount of external 

 mucus. 



1 It is to be recommended at the next International Botanical Congress that the generic name 

 Sirogonium be retained. 



W. A. 23 



