FUCALES (HETEROGENERATAE) 197 



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

 bodies, e.g. Laminaria. 



(b) Plants with hairs and reproductive bodies in scattered sori, 

 e.g. Dictyota. 



(c) Plants with hairs and reproductive bodies in scattered re- 

 ceptacles, e.g. Durvillea. 



(d) Plants with hairs and reproductive bodies in receptacles 

 which are confined to apical positions or special side branches, e.g. 

 Fucus, Ascophyllum. 



In the mature fruiting conceptacles there are branched hairs or 

 paraphyses with the microsporangia borne terminally on the 

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

 associated with the megasporangia, which are either sessile or else 

 borne on a single stalk cell, each megasporangium characteristically 

 containing eight ova when mature. In those species where the 

 conceptacles are hermaphrodite all these structures occur together. 

 The walls of both sporangia are double, and when the gametes are 

 ripe the sporangia burst, liberating their contents which are still 

 enclosed in the inner delicate membrane. The expulsion of the 

 gametes normally takes place whilst the tide is out because the con- 

 ceptacle 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, whilst the 

 inner megasporangium wall inverts and enables the ova to escape. 

 Fertilization takes place in the sea, the antherozoids clustering 

 around the ova and causing them to rotate by their activity until 

 one antherozoid succeeds in entering and fertilizing each ovum. 



The fertilized zygote surrounds itself with a wall and very shortly 

 begins to divide, the direction of the first wall being said to be at 

 right angles to the incident light. After a few more divisions the 

 octant stage is reached and then a rhizoid appears on the side away 

 from the light and grows downward, being followed soon after by 

 others (cf. p. 289). The upper part of the embryo elongates from a 

 five-sided apical cell but the end soon becomes flattened, after 

 which a terminal depression arises that contains the three-sided 

 juvenile apical cell together with a bunch of hairs. The bunch of 

 hairs possess trichothallic growth, but they soon fall oflF and the 

 basal cell of one hair becomes the new four-sided apical cell of the 



