BOTANY OF THE LIVING PLANT 



tetrad in Land Plants is obvious. But here the haploid gametophyte and the 

 diploid sporophyte are alike in form. 



In contrast to the isomorphic alternation seen in the Ectocarpales, Dictyotales, 



the Laminariales afford remarkable examples of fc alternation. 



In this Order the large plant itself— for example I rim as already 



:ribed. Fig. 2S1— is the diploid sporophyte : the gametophyte, by contrast, 



minute, filamentous plant. In club-shaped, unilocular sporangia, borne on 



roush patches on the frond of Laminaria, reduction takes place and haploid 



zoospores are liberated. These give rise to minute, filamentous male and 



female gametophytes Fif - --6) ; the former are branched and bear antheridia 



while the latter consist of a few cells, and in extreme cases may be reduced 



to a single cell, and bear oogonia. The naked egg-cell which emerges from the 



oogonium and remains attached thereto, grows into the diploid sporophyte on 



fertilisation. 



The Fucales are conspicuously oogamous. The sexual organs are in 

 conceptacles, cavities hollowed out of the thallus (Fig. 280). Fucus 

 serratus (Common Wrack: bears the male and female organs on distinct 

 plants, but in many species they may appear in the same conceptacles. 

 A median section through a male conceptacle shows how the flask- 

 shaped cavity opens to the outside by a narrow pore, and is at 

 maturity filled by richly branched hairs, which arise from the tissues 

 bounding the conceptacle, and bear the numerous minute antheridia, 

 or nude ganietangia (Fig. 282). The female conceptacle is of like 

 structure, but bears unbranched hairs, and associated with them are 

 the large oogonia, or female gametayigia, which are large enough when 

 mature to be seen with the naked eye (Fig. 283). 



The antheridium itself is an oval unicellulcr body, surrounded by 

 a cell-wall. When young it contains a single nucleus, which divides 

 to form 64 nuclei, each of which becomes the centre of a spermatozoid. 

 The cytoplasm divides into as many portions, and each is found to 

 contain a red eye-spot beside the nucleus (Fig. : 9 ~. : . The contents 

 slip out when ripe from the ruptured outer wall, as a mass still 

 surrounded by the inner wall : this soon deliquesces, and sets them 

 free as 64 motile sperr .::;:: .;: each with the characters usual for 

 the Brown Seaweeds (3, 4). The oogonia though larger are of the same 

 pattern. Each has at first one nucleus ; but here it divides to form 

 only eight, and the cytoplasm undergoes cleavage into eight large 



• 2. They are also shed in the same way, and round themselves 

 off as 8 non-motile eggs (5). [See also Fig. 288, D.E.) 



A comparison of the antheridium with the oogonium in the Fucaceae shows 

 that they are probably results of differentiation from a common source. When 

 an oogonium is to be formed, a cell of the wall of the conceptacle projects with- 

 out branching into the cavity*, and divides to form a stalk-cell and an 



