258 



PART III. THE CLASSIFICATION OF PLANTS. 



The structure of the cells of Cbromophyton, and its modes of reproduction, 

 are essentially the same as those of Hydrurus. 



The Syngenetica3 somewhat resemble the palmelloid Protococcaceae among 

 the Chlorophyceae, and may be regarded as the corresponding group in the 

 Phaeophyceae. They grow only in fresh water; Hydrurus in cold, rapidly running 

 streams. 



Order 2. Diatomaceae. Unicellular plants, either free, or connected into 

 filaments or masses by mucilage ; sometimes attached. Eeproduction, vegeta- 

 tive by division; or by means of asexually produced spores; or sexual isogamous 

 by the conjugation of aplanogametes. The cell-wall is impregnated with silica. 

 Both fresh-water and marine. 



The Diatomaceae resemble the Desmidieae in many respects ; the two orders, 

 are, in fact, corresponding forms in the Phaeophyceae and the Chlorophyceae 

 respectively ; but, besides their colour, the Dia- 

 toms differ from the Desmids in the presence of 

 silica in their cell-wall. 



The cell, or frmtule, as it is called, is enclosed 

 by a rigid wall. The wall, like that of the Desmids, 

 consists of two halves, called valves, of different 

 ages, which are not directly continuous, but are 

 j elated to each other as the two parts of a pill- 

 box, the one overlapping the other (Fig. 184). 

 The cell-contents consist of a more or less vacuo- 

 lated mass of protoplasm, which forms a layer in 

 close contact with the inner surface of the cell- 

 wall ; in this there is a nucleus, sometimes pari- 

 etal sometimes central, and chromatophores 

 which may be very numerous and small, or few in 

 number (sometimes only one) in the form of re- 

 latively large plates. 



Vegetative multiplication by division takes place 

 by the division of the protoplasm into two cells ; 

 each of these cells has one of the two valves of the 

 parent frustule on one side of it ; it then secretes 

 a new valve on its naked side, which is smaller 

 the older valve and fits inside its rim ; thus two new individuals are 



Ul 



FIG. 184. Pinnularia, a Dia- 

 tom (mag. and diag.) ; a lateral 

 view, showing the mode of 

 connection of the two halves 

 of the frustule ; s surface view. 



than 

 formed. 



It will be noted that this process of multiplication is accompanied by a de- 

 crease in size ; and, were it repeated indefinitely, the cells would become very 

 small. This process of diminution is interrupted by the formation of auxo- 

 spures, either asexually or sexually. In the former case the protoplasmic con- 

 tents of a cell escape from the separated valves, as an auxospore, which, after 

 growing considerably, secretes two new valves forming a new and larger frustule. 

 In the latter case, two naked cells which have thus escaped, conjugate to form 

 an auxospore which becomes a new frustule. This process of conjugation 

 differs, however, from that of the Desmidieas, in that no resting zygospore is 

 formed, but simply a new individual. 



Series PH^OSPORE^:. The multicellular body is attached ; it sometimes con- 



