548 MICROSCOPIC FORMS OF VEGETABLE LIFE THALLOPHYTES 



controversy. Professor Cohn : observed that the oscillating move- 

 ments take place only when the thread is in contact with a solid 

 substratum. Zukal 2 compares the motion of Xj>i/-n/ina to that of a 

 growing tendril, and asserts that it is intimately connected with the 

 growth of the filament. Hansgirg, 3 on the other hand, considers the 

 twisting and nodding movements to be due, not to the growth of the 

 thread, but to osmotic changes in the cell-contents. He regards them 

 as being of the same nature as the movements of the sarcode in the 

 pseudopodia of rhizopods and other protozoa. Schnetzler 4 describes 

 the movements in Oscillatoi i as of six different kinds : (1) rotation of 

 the thread or of its segments round its axis ; (2) creeping or gliding- 

 over a solid substratum ; (3) a free-swimming movement in the water : 

 (4) I'otation or flexion of the entire thread ; (5) sharp tremblings or 



concussions ; and (6) a radiating arrange- 

 ment of the entangled threads. The 

 movements are greatly influenced by 

 temperature and light, being much more 

 active in warmth and sunshine than in 

 cold and shade. There are no zoospores 

 produced, nor is any sexual mode of 

 generation known. The Rivulariacece 

 and Scytonemacece (Pis. YII and VIII) 

 are exceedingly common organisms in 

 stagnant water, resembling the Oscilla- 

 toriacete in their blue-green colour, and 

 in their reproduction by means of 



' hormogoiies.' 



PIG. 419. Poitioii of gelatinous 

 frond of Nostoc. 



Nearly allied to the preceding is the 

 family of Nostocacece, consisting of 

 distinctly beaded filaments, which, in 

 the most familiar genus, Nostoc, lie in 

 firmly gelatinous envelopes of definite 

 outline (fig. 419). The filaments are 

 usually simple, though sometimes densely 

 interwoven, and arc almost always curved 

 or twisted, often t a king a spiral direction. 



The masses of jelly in which they are imbedded are sometimes 

 globular or nearly so, and sometimes extend in more or less 

 regular branches; they frequently attain a very considerable 

 size ; and as they occasionally present themselves quite suddenly 

 (especially in the latter part of autumn on damp garden-walks), 

 they have received the name of 'fallen stars.' They are not 

 always so suddenly produced, Imuever. as they appear to lie: for 

 they shrink up into mere films in dry weather and expand again 

 with the first, shower. Other species are not unfrequenl among wet 

 QlOSS or, OD the surface of damp rocks. Species of A nl><i>ini and 

 Aphanizomenon, genera of Xostocacea 1 . const it ute a large portion of 



1 Arc 1 '. .I///,- ms/,:. Anatomie, lsr>7, ]>. -18. 

 - Oesterreichische />'<>/. /.I'itscln: IHsti, p. 11. 

 5 See Uut. Cnilmllihilt, vol. xii. Lss-2, ].. :;r,i. 

 1 Arch. Sci. Phys. et Nat. 1885, \<. if.i. 



