CHAPTER TV. THE THALLOPHYTA. 277 



They are all chlorophylless organisms, with nearly transparent 

 cell-contents. In all putrefying fluids, or solutions that contain 

 decaying organic matters, they swarm in myriads. They are, in 

 fact, the inciting cause of putrefaction. By their agency also 

 milk sours, wine is converted into vinegar, and many other 

 important fermentations take place. So far as animal life is 

 concerned, some of the species are harmless or perhaps even 

 beneficial, while others are the source of some of the most 

 dreaded and most fatal of diseases. Chicken cholera, splenic 

 fever, small-pox, diphtheria and leprosy are diseases of this 

 class. A peculiar interest, therefore, attaches to the study of 

 these organisms. 



The cells agree in having mostly rigid, transparent walls and 

 colorless cell-contents, but different species differ considerably 

 in form, size and in conditions and habits of growth. Their 

 usual mode of increase is by fission, but they also produce very 

 minute spores by internal cell-formation. In some species the 

 cells, after fission, immediately become independent ; in others 

 they remain united for a time to form filaments or chains of 

 various length. 



Many of the species, in some stage of their development, 

 have the habit of secreting a jelly and increasing rapidly by 

 fission, forming large gelatinous colonies. These are called 

 zooglcea-mdisses. The " mother-of-vinegar " and the so-called 

 "blood-rain," consisting of red gelatinous spots, often found on 

 putrefying bread, are illustrations. 



Most of the species have both their quiescent and their 

 motile stages of development. 



There is no group of plants in which it is at present so diffi- 

 cult to define the limits of species. It is difficult to classify 

 them by their forms, because these, under certain conditions, 

 have been found to change into others quite different, and a 

 similar objection applies to classifying them according to their 

 physiological effects, since it has been found that, in some 

 instances at least, an innocent species may, by cultivation, be 

 changed to one of great virulence and, vice versa, a. virulent one 

 may be changed to one that is harmless. For practical purposes, 

 however, Cohn's classification, based on the forms, is as con- 

 venient as any. According to this author there are four princi- 



