CHAPTER V. THE THALLOPHYTA. 28 1 



CHAPTER V.— THE THALLOPHYTA (Continued). 



Class III. — The Alg.«. 



THE DIATOMACEiE AND THE CHLOROPHYCEiE. 



Under the Algae are included that vast host of Thallophytic 

 plants which contain chlorophyll, except the Cyanophyceae, 

 which have already been treated, and the Lichenes, which con- 

 sist of chlorophyll-bearing, associated with chlorophylless Thal- 

 lophytes. The group is divided into four sub-classes, the Diato- 

 macece, the Chlorophycecz, the Melanophycece- and the Floridea. 



THE DIATOMACEiE. 



The Diatomaceae are so peculiar irt their structure that 

 some botanists have relegated them to a class by themselves. 

 They are all in reality unicellular, though sometimes united in 

 colonies, and all are microscopic in size. They possess chloro- 

 phyll-plates in their protoplasm, but the green color is more or 

 less obscured by the presence of a peculiar brown coloring mat- 

 ter. They are also endowed with the power of locomotion, the 

 movement being a gliding one, but how it is accomplished is not 

 yet certainly known. 



The great distinguishing feature of the group, however, is the 

 peculiar structure of the enclosing membrane. This is a silicious 

 box consisting of two pieces fitting one into the other, like the 

 parts of a common band-box. The two valves, as the parts of 

 the box are called, are usually alike, excepting that one is a trifle 

 larger than the other, so as to fit over it, and both are beauti- 

 fully and often very delicately and regularly sculptured. 



Not less interesting and strange is their mode of reproduc- 

 tion. This takes place by fission, and in many species also 

 sexually by conjugation, but the processes are peculiar. When 

 the process of fission begins, the valves separate slightly from 

 each other, the protoplasm divides into two portions, and each 

 secretes for itself a new valve to fit within the old one that lies 

 adjacent to it, and the plants thus become independent. 



It is evident that, as successive divisions take place, there 

 will be a gradual reduction in the size of the plants, since the 

 rigid valves once formed are not capable of expansion ; so the 



