242 PART III. THE CLASSIFICATION OF PLANTS. 



Vegetative propagation occurs in some forms : an entire segment (Khizo- 

 clonium), or a part of a segment (Pithophora), or several parts of a repeatedly 

 septated segment (Urospora) become densely filled with protoplasmic contents 

 and then break away from the body, and germinate either at once or after a 

 period of rest ; in Urospora this body, which appears in all cases to be of the 

 nature of a gemma, sometimes gives rise to zoogonidia. 



Cladophora and Chsetomorpha occur in both salt and fresh water ; Pithophora 

 exclusively in fresh water ; Urospora exclusively in salt water ; Ehizoclonium 

 occurs both in fresh and salt water, and also in damp soil. 



Fam. 3. Valoniea : body, essentially tubular and much branched, attached 

 by a root, the thalloid shoot presenting various forms (vesicular or clavate, or 

 a flattened expansion), with apical growth ; the only form of reproduction 

 known is asexual by zoogonidia (in Vaionia, Siphonocladus, Anadyomene) : 

 principal genera, Vaionia, Dictyosphseria, Siphonocladus, Struvea, Anadyomene ; 

 all marine. 



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

 ccenobium, formed by the aggregation of originally distinct cells, of limited 

 growth; a net (Hydrodictyon), or a flat plate (Pediastrum), or a solid sphere 

 (Sorastrum), or a hollow sphere (Coelastrum) : reproduction, asexual by zoo- 

 gonidia (Hydrodictyon, Pediastrum) or non-motile gonidia (Coelastrum, Sora- 

 strum) ; sexual, isogamous by planogametes (known only in Hydrodictyon and 

 Pediastrum). 



It appears that Coelastrum and Sorastrum are really cellular plants, each 

 segment of the body being a cell with a single nucleus. 



All four genera 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 zoogonidia in any one of the segments of the ccenobium ; the 

 zoogonidia do not escape from the segment (gonidangium), 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 gonidangium, and then grows to the full size. The sexual re- 

 production consists in the formation in a segment (gametangium) of the 

 'ccenobium, 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 tljick wall and is angular in form, undergoes a period of quiescence ; 

 on germination 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 sur- 

 rounds itself with a thick cell-wall, and assumes a peculiar angular form, on 

 account of which it has been termed the polyhedron-stay e. 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 themselves into a small Hydro- 

 dictyon-plant. Thus the life-history of Hydrodictyon presents a definite alter- 

 nation of generations ; the plant is the gametophyte ; the zygospore and the 

 polyhedron together represent the sporophyte. 



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

 but in Pediastrum the zoogonidia are set free from the gonidangium, sur- 



