m 



BOTANY OF THE LIVING PLANT 



repeated divisions to form short discoid cells, which are the antheridia. 

 Each, on opening in the same way, sets free two spermatozoids, the 

 result of division of its protoplast. They resemble the zoospores 

 in form, but are smaller. In some species special small plants (dwarf- 

 males) are produced from smaft zoospores, which fix themselves in the 

 neighbourhood of the oogonium, and dividing into a few cells, liberate 

 their spermatozoids close to the opening (Fig. 331, C, D). Fertilisa- 

 tion follows by fusion of the spermatozoid at the receptive spot of 

 the ovum, and the coalescence of the nuclei has 

 been observed. The zygote formaS a firm pro- 

 tective wall : it is stored with nutriment, takes 

 a brown or red colour, and may enter a period 

 of rest. Its germination presents a point of 

 special interest. The outer wall bursts and the 

 contents escape, contained within a delicate 

 membrane. The protoplast then divides usually 

 into four cells, which ultimately escape as 

 motile zoospores (Fig. 332). This division into 

 four, following on sexual fusion, suggests a 

 process of reduction comparable to that seen 

 in the tetraspores of Polysiphonia or Dictyota, 

 or in the spore-mother-cells of Mosses and 

 But it is stated that the zygote may occasionally germinate 



Fig. 332. 

 Bulbuchaete intermedia. 

 A -^ oospore. B = formation 

 of four zoospores in the 

 gerimnating oospore. (After 

 Pringsheim, from Strasbur- 

 ger.) ( X 250.) 



without those divisions, to form a new individual directly, while an 

 unfertilised oogonium may do the same. It would thus appear that 

 the cycle of events is not so stereotyped here as it has become in more 

 advanced plants. 



SiPHONALES. 



The Siphonales have already been discussed in Chapter IX., in respect 

 of their pecuhar structure (p. 139, Fig. 98). They are coenocytes : 

 that is, they are not partitioned into separate cells, but the plant-body 

 consists of a large non-septate sac, limited by a cell-wall, and kept 

 firm enough by internal turgor to preserve its form in the quiet water 

 in which as a rule they live. This condition is shown in a less complete 

 form in the Siphonocladiales, where septa occur at intervals, but 

 the protoplasts lying between these are multi-nucleate, and hardly 

 warrant their designation as cells. This is seen in Cladophora, a 

 very common genus of fresh and salt-water Algae. Clearly the 

 non-septate state is mechanically ineffective. It has been pointed 



n 



