332 



BOTANY 



its cilia, and rounding itself off, becomes converted into an egg, which, after its 

 fertilisation by a microgamete, is invested with a wall and forms a resting zygote. 



Order 2. Fueaeeae 



Asexual reproduction is wanting in this order, while sexual reproduction is 

 distinctly oogamous. The oogonia and antheridia, as in Fucus vesiculosus and 

 platycarpus, for example, are formed in special flask-shaped depressions termed 

 conceptacles, which are crowded together below the surface in the swollen tips or 

 receptacles of the dichotomously branched thallus (Kg. 253,/). The conceptacles 

 of F. platycarpus (Fig. 257) contain both oogonia and antheridia, while F. 



vesiculosus, on the contrary, is dioeci- 

 ous. From the inner wall of the con- 

 ceptacles, between the oogonia and 

 antheridia, spring numerous, un- 

 branched, sterile hairs or pabaphyses, 

 of which some protrude in tufts from 

 the mouth of the conceptacle (Fig. 

 257). The antheridia are oval in 

 shape, and are formed in clusters 

 on special short and much-branched 

 filaments (Figs. 257 a, 258 <J). The 

 contents of each antheridium sepa- 

 rate into a large number of sperma- 

 tozoids, which are discharged in a 

 mass, still enclosed within the inner 

 layer of the antheridium (Fig. 258, 

 B). Eventually set free from this 

 outer covering, the spermatozoids ap- 

 pear as somewhat elongated, ovate 

 bodies, having two lateral cilia of 

 unequal length and a red eye-spot (ff). 

 The oogonia (Fig. 257, o) are nearly 

 spherical, and are borne on a short 

 stalk consisting of a single cell. They 

 are of a yellowish-brown colour, and 

 enclose eight spherical egg-cells which are formed by the division of the oogonium 

 mother cells. The eggs are enclosed within a thin membrane when ejected from 

 the oogonium (Fig. 258, A). This membranous envelope deliquesces at one end 

 and, turning partly inside out, sets free the eggs. The spermatozoids then gather 

 round the eggs in such numbers that by the energy of their movements they often 

 set them in rotation (F, E). After an egg has been fertilised by the entrance of 

 one of the spermatozoids it becomes invested with a cell wall, attaches itself to 

 the substratum, and gives rise by division to a new plant. In the case of other 

 Fueaeeae which produce four, two, or even only one egg in their oogonia, the nucleus 

 of each oogonium, according to Oltmanns, nevertheless first divides into eight 

 daughter nuclei, of which, however, only the proper number'give rise to eggs 

 capable of undergoing fertilisation. The oogonia, accordingly, of the Atlantic 

 Himanthalia lorea, which produces only one egg, just as those of other species in 

 which two or four are developed, may be regarded as having been evolved phylo- 

 genetically from oogonia in which eight eggs are formed. 



ci o o 



Fig. 257. — Fucus platycarpus. Monoecious con- 

 ceptacle with oogonia of different ages (o), and 

 clusters of antheridia (a) ; p, paraphyses. (After 

 Thuret, x circa 25.) 



