BROWN ALGAE 387 



ovum (Fig. 324, 6). The rest at once move away, as though a repellent 

 influence from the egg had replaced the previous attraction. 



The immediate consequence of fertilisation is the deposit of a 

 cell-wall covering the zygote. It settles on some solid substratum, 

 and germinates directly into a new Fucus plant. Stones on a rocky 

 shore where Fucoids grow may be found in summer covered by a 

 dense growth of myriads of these young plants (Fig. 326). 



In the life-history of Fucus the increase in numbers is exclusively through 

 the sexual process. There is no production of spores, nor any non-sexual 

 propagation, as there is in the simpler Brown Seaweeds. But in Dictyota, 

 which is grouped with the Fucaceae in the Cyclosporeae, there is an alter- 

 native production of tetraspores. Separate plants in Dictyota dichotoma 

 bear respectively tetraspores, oogonia, and antheridia. The nuclei in 

 the first of these plants are diploid, with 32 chromosomes. The nuclei 

 of the male and female plants are haploid, with only 16 chromosomes. 

 Reduction takes place when the tetraspores are formed, and these on 

 germination give the sexual plant. The analogy of the tetraspores with 

 the spore-tetrad in Land Plants is obvious. But here the haploid gameto- 

 phyte and the diploid sporophyte are alike in form. In Fucus reduction 

 takes place in the first divisions respectively of the antheridial and oogonial 

 cells. The whole plant is a diploid sporophyte, arising from the germination 

 of the diploid zygote. Such facts raise questions as to the origin of the 

 alternate generations seen in Dictyota, or their absence in Fucus. But these 

 can only receive their final answer after exhaustive examination of the 

 nuclear cycle in the simpler members of the series. In them the details are 

 still unknown. 



It seems clear from the peculiarity and the constancy of form of 

 their propagative cells that the Brown Seaweeds are a naturally 

 related group of organisms. They illustrate steps of advance in 

 respect of form, structure, and propagative method ; and these 

 follow for the most part along parallel lines. Accordingly they 

 may be held to represent a progressive series. That they are ranked 

 as Thallophytes is no sufficient reason for holding them all as primitive. 

 Their higher terms show high structural adaptation to their require- 

 ments, and they are in their own habitat eminently successful plants. 

 They may therefore be held as the ultimate exponents of an evolution 

 limited by its surroundings, and distinct from other lines of Descent. 



RED ALGAE (RHODOPHYCEAE}. 



The Red Algae (Rhodophyceae) are a separate group, distinguished by 

 their method of propagation from all others. They are mostly marine, 

 spreading from the zone between tide-marks to deeper levels, and finding 



