34 FIJiST GROUP. — THALLOPHYTES. 



others straight {V. sericea), or like a bent bag or pouch ( V. pachyderm a). In V. synan- 

 dra, discovered by Woronin, from two to seven small horns are formed on the large ovoid 

 terminal cell of a two-celled twig. The oogonia are thicker protuberances, densely 

 filled with oil and chlorophyll [og in Fig. 15, A and B)\ they swell usually into an 

 obliquely ovoid form, and their contents are at length cut off by a transverse wall 

 (F, osp). The green coarse-grained mass collects in the centre of the oogonium, while 

 a colourless protoplasm gathers at its apex, where the oogonium opens ; at this moment 

 the whole of the contents contract and form the oosphere, and in some species a colourless 

 slime is expelled from the opening at the apex. After the entrance of the spermatozoids 

 the oosphere invests itself with a thick wall, its contents become red or brown, and the 

 oospore now begins its period of rest. The formation of the oogonia and antheridia 

 begins in the evening, is completed during the next morning, and fertilisation ensues 

 between ten and four o'clock of the same day. 



It has lately been ascertained ^ that Vaucheria passes through resting stages of like 

 character with those of Botrydiinii. The plant in these stages used to be described as 

 a distinct genus under the name of Gongrosira. The tubes are divided by thick gela- 

 tinous cross-walls into a number of separate segments (cells), which after the close of the 

 period of rest either develope each into a vaucheria-tube, or their protoplasm divides 

 into a number of separate pieces, which issue forth in a body invested with a thin 

 pellicle or an envelope of mucilage, and then the several portions become isolated. 

 They are not however swarm-spores as in Botrydiitui ; but they creep about with 

 constant change of shape on the substance on which they have settled, just like amoebae. 

 Each of these amoebiform bodies rounds itself off into a green sphere and invests itself 

 with a membrane, and either germinates and developes directly into a vaucheria-tube, 

 or can again enter upon a period of rest. 



2. THE VOLVOCINEAE -. 



The Volvocineae consist of cells that are either isolated or united together in 

 gelatinous envelopes to form families {coembid). These coenobia are either hollow 

 spheres, as in Volvox and Eudorina, or four-cornered plates, as in Gonium ; and 

 though surrounded by a cell-wall they have the power of independent motion, for 

 each cell has two long cilia which protrude through the cell-wall. The isolated cells 

 of Chlamydomonas and Chlamydococcus swim about by this means like ordinary 

 swarm-cells ; in the coenobia the cilia of all the individual cells project beyond the 

 common envelope, and their united efforts give a movement of rotation to the whole 

 coenobium. 



The differences in the mode of sexual propagation are still more striking than 

 those in the structure of the plants themselves. 'The simplest case is where two 

 swarm-spores of similar form unite with one another, as in Pandorma and species of 

 Chlamydomonas ; in other cases, as in Eudorina and Volvox, the male element is 

 clearly distinguishable from the female. 



' Stahl, Ueber die Ruhezustande der V. geminata (Bot. Zeit. 1879, P- ^29). 



^ Fringsheim, Ueber Paarung der Schwarmsporen (Munatsber. der Berliner Akad. Okt. 1869). — 

 Cohn, Entvvicklungsgesch. der Gatt. Volvox ;Cohn's Beitr. z. Biol. d. Pflanz. Bd. I). — Kirchner, Zur 

 Entw. des Volvox minor (Cohn's Beitr. z. Biol. d. Planz. Bd. III). — Goroshankin, Die Genesis bei 

 den Palmellaceen (Referat iiber diese Abhandl. in Bot. Jahresbericht, 1875'. — Rostafinski, Quelques 

 mots sur V Haei)ialococciis lacustris (Mem. de la Soc. nat. d. sc. nat. de Cherbourg, 1875, T. XIX). — 

 Cohn u. Wichura, Ueber Stephanosphaera pluvialis (Nova .A.cta Leop.-Carol. Vol. XXVI,. — De 

 Bary, Bot. Zeit. 1853, Beilage, p. 73. — Rostafinski, in Bot. Zeit. 1871, p. 757. 



