22 



THE AMERICAN MONTHLY 



[February, 



more fully ; but since the literature of 

 the subject is so meagre and scat- 

 tered, comparatively little can be said 

 about the life-history of swarm-spores. 

 From what has been already said of 

 the production of swarm-spores, it will 

 be understood that these are not 

 usuall}', the product of any process of 

 conjugation. They are purely a sexual 

 elements. In a few cases such swarm- 

 spores have been observed to conju- 

 gate after emission ; but this is a rare 

 occurrence, and whether it is indica- 

 tive of any distinction between the 

 coalescing gonidia is uncertain, and, 

 it may be added, improbable. 



The plant Ulothrix, which is very 

 common about here, propagates by 

 the production of a single swarm- 

 spore in each cell. This plant is 

 represented in fig. 6, c, with a swarm- 

 spore just making its exit from a 

 cell, and another swimming away 

 from the empty cell at b. I have 

 often watched these swarm-spores 

 making their exit in clouds from a 

 collection of Ulothrix in a saucer. 

 Usually I have observed them to 

 grow directly into new filaments ; but 

 probably they sometimes become 

 spherical and surrounded by jelly, 

 and multiply in that condition. Mr. 



;FiG. 6. — a, ChjEtophora ; h, Glceocapsa; c, Ulothrix; d, Nostoc; e, Scytonema ; _/, Sirosiphon. 



Among the Phycochromophyceae, 

 or dark, olive-green algae, there are 

 classed a number of unicellular plants 

 under the generic name of Glceocapsa, 

 of which Rabenhorst describes over 

 fifty species. This alga consists of 

 single, spherical cells, surrounded by 

 a wide, mucilaginous layer, showing 

 numerous concentric rings (fig. 6, b). 

 These cells seem capable of multi- 

 plying indefinitely; but under some 

 conditions, according to the Rev. Mr. 

 Wolle's observations, they develop 

 into filamentous plants, as Sirosiphon, 

 ,(fig. 6,y"), or Scytonema (fig. 6, e). 



Wolle believes that the green cells 

 found growing on board fences, tree- 

 trunks, and in similar places, which 

 are known as Pleurococcus, are merely 

 the spores of Ulothrix. This pulver- 

 ulent Pleurococcus consists of small 

 masses of si^herical cells. I have 

 been studying it for some time, hoping 

 to induce the ulothrix-filaments to 

 grow ; but up to this time, they have 

 not been produced. 



Another plant known as Nostoc (fig. 

 6, d), Mr. Wolle believes to be a de- 

 velopmental form of Scytonema, which 

 may be regarded as a Nostoc en- 



