GROUP I. THALLOPHYTA : ALG.E. 



265 



Order Laminariacese (Laminaria, Alaria, Chorda, etc.). 

 Order Cutleriacese (Cutleria Zanardinia). 



Series (l>) PH^OGAM^E : sexual reproduction oogamous; asexual, want- 

 ing, or by non-motile spores. 

 Order Fucacese. 



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

 filaments or masses by mucilage; sometimes attached. Reproduction, 

 vegetative by division; or by means of asexual ly 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 Desmidiese in many respects; the two 

 orders are, in fact, corresponding forms in the Phseophycese and the 

 Chlorophycese respectively ; but, besides their colour, the Diatoms differ 

 from the Desmids in the presence of silica in their cell-wall. 



The cell, or frustule, 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 ? whiph 

 are not directly continuous, but are related to 

 each other as the two parts of a pill-box, the 

 one overlapping the other (Fig. 152). The 

 cell-contents consist of a more or less vacuo- 

 lated mass of protoplasm, which forms a laj-er 

 in close contact with the inner surface of the 

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

 parietal sometimes central, and chromato- 

 phores which may be very numerous and 

 small, or few in number (sometimes only one) 

 in the form of relatively 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 than the older valve and 

 fits inside its rim; thus two new individuals 

 are formed. 



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

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

 very small. This process of diminution is interrupted by the formation 

 of auxospores, either asexually or sexually. In the former case the pro- 

 toplasmic contents of a cell escape from the separated valves, as an auxo- 

 spore, which, after growing considerably, secretes two new valves forming 

 a n.-w 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 

 Desmidiese, in that no resting zygospore is formed, but simply a new 

 individual. 



Series PH^EOSPOEE^. The multicellular body is attached; it some- 



FiG.162. Pinnularia, a Dia- 

 tom (mug. and diag.); lateral 

 view, showing the mode of 

 connection of the two halves 

 of the frnstule; s surface view. 



