202 THE ALGAE 



no relation to the sexual conceptacles but have developed with them 

 in a parallel manner. On the whole the available evidence supports 

 the first hypothesis, and on this basis the following morphological 

 series can be arranged : 



(a) Plants with a continuous patch of hairs and reproductive 



bodies, e.g. Laminaria (see p. 178). 

 ih) Plants with cryptostomata and continuous patches of hairs 



and unilocular sporangia, cf. Alaria (see p. 189). 

 (c) Plants with cryptostomata containing hairs and sporangia, 



e.g. Splachnidium rugosum, Ascoseira mirahilis (see p. 318). 

 {d) Plants with hairs and reproductive bodies in scattered con- 

 ceptacles, e.g. Durvillea. 

 (e) Plants with hairs and reproductive bodies in conceptacles 

 which are confined to special positions (receptacles), e.g. 



Fucus. 



In the mature fruiting conceptacles there are branched hairs or 

 paraphyses with the antheridia borne terminally on the branches 

 near the base, or else the paraphyses are unbranched and associated 

 with the oogonia, which are either sessile or else borne on a single 

 stalk cell, each oogonium characteristically containing eight ova 

 when mature. In those species where the conceptacles are herma- 

 phrodite all these structures occur together. The wall of the oogon- 

 ium (macrosporangium) is three-layered (see p. 193) but that of the 

 antheridium is two-layered. The explusion of the gametes normally 

 takes place whilst the tide is out because the conceptacle is then 

 full of mucilage and the loss of water causes the thallus to shrink, 

 thus forcing the ripe ova and antherozoids in their envelopes 

 through the ostiole to the surface. When the tide returns the inner 

 wall bursts and so liberates the antherozoids. Each oogonium wall 

 has three layers which are called the endochiton, mesochiton and 

 exochiton. Normally when the ova are ripe the outer exochiton 

 ruptures and liberates the eggs enclosed in the meso- and endo- 

 chiton. Next the mesochiton ruptures and inverts itself, thus 

 exposing the endochiton which is finally dissolved and the eggs set 

 free into the water where fertiUzation takes place, the antherozoids 

 clustering around the ova and causing them to rotate, until even- 

 tually one penetrates and fertihzes the ovum. A fertihzation mem- 

 brane is then produced. The entire process can usually be demon- 

 strated in the laboratory. 



The fertiHzed zygote divides at first by three transverse walls and 



