GROUP i. THALLOPHYTA: ALG^E. 253 



all or any of the segments of the body without special modification ; the 

 zygospore appears to develope directly into a new plant (Cladophora). 



Cladophora and Chaetomorpha oocur in both salt and fresh water ; 

 Pithophora exclusively in fresh water ; Urospora' exclusively in salt 

 water ; Rhizoclonium occurs both in fresh and salt water, and also on 

 damp soil. Cladophora is to be found attached to stones in ditches. 



Order 3. Hydrodictyaceae : body thalloid. a non-motile unattached 

 c^nobium, formed by the aggregation of originally distinct cells, of 

 limited growth ; a net (Hydrodictyon), or a flat plate (Pediastrum) : 

 reproduction, asexual by zoospores ; sexual, isogamous by planogametes. 

 These plants are confined to fresh water. 



The following is a brief sketch of the life-history of Hydrodictyon. 

 The asexual reproduction of this plant consists in the formation of a 

 large number (7,000-20,000) of zoospores in any one of the segments of the 

 coenobium ; the zoospores do not escape from the segment, but swim 

 actively within it for a time, when they come to rest, cohering, as they 

 do so, to form a small net-like coenobium, which is eventually set free by 

 the disorganisation of the wall of the sporangium, and then grows to 

 the full size. The sexual reproduction consists in the formation in a 

 segment (gametangium) of the coenobium, of a very large number 

 (30,000-100,000) of small planogametes ; these are set free into the water, 

 and conjugate to form zygospores. The zygospore, which has a thick 

 wall and is angular in form, undergoes a period of quiescence ; on germi- 

 nation its contents divide into two or more cells which are set free as 

 zoospores, and, after a brief period of motility, come to rest. Each then 

 surrounds itself with a thick cell-wall, and assumes a peculiar angular 

 form, on account of which it has been termed the polyhedron-istage. The 

 polyhedron grows and its contents divide into a number of zoospores ; the 

 outer coat of the polyhedron then ruptures, and the contents, surrounded 

 by the thin inner coat, are extruded ; the zoospores then arrange them- 

 selves into a small Hydrodictyon-plant. 



The life-history of Pediastrum is essentially the same as that of Hydro- 

 dictyon ; but in Pediastrum the zoospores are set free, surrounded by a 

 delicate membrane within which they come to rest and cohere to form a 

 Pediastrum plant. 

 Series IV. CONFERVOIDE.S:. The principal forms included in this series 



may be arranged in the following orders : 

 Sub-series A, Azoosporese : no zoospores. 



Order 1. Conjugatse : sexual reproduction by aplanogametes. 

 Sub-series B. Zoosporese ; asexual reproduction by zoospores. 

 Itiogamous Orders : 



Order 2. U lothrichaceae : body filamentous, unbranched. 

 Order 3. Ulvaceae : body a flat or tubular cellular expansion. 

 Oogamous Orders : 



Order 4. CEdogoniaceae : body filamentous, unbranched (except Bulbo- 

 chsete). 



Order 5. Coleochsetacae : body filamentous, branched ; oogonium with 

 a trichogyne. 



