388 BOTANY OF THE LIVING PLANT 



that in the Brown Algae there has been a progressive retrogression of the 

 haploid phase culminating in the condition found in Fucus. In this view, the 

 first two nuclear divisions in the antheridium and oogonium are regarded as a 

 tetrad division within a sporangium ; the subsequent developments are held 

 to represent all that remains of the greatly reduced gametophyte generation. 

 Such conceptions, should, however, be viewed with caution. An alternative 

 hypothesis, for which there is evidence among several of the less specialised 

 groups, is that the reproductive mechanism in the Fucales had its origin in a 

 tendency for the asexual zoospores (produced in unilocular sporangia) to 

 behave as gametes. As the Fucales are undoubtedly advanced forms with a 

 long history behind them, the matter under discussion is clearly one which 

 calls for a suspension of judgment until further data are available. 



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

 their motile propagative cells that the Brown Seaweeds are a natur- 

 ally 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. 

 These 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 

 their limit at about 150 feet below low-water mark. Their colour varies 

 from pink to purple, or reddish brown. This is due to chromatophores 

 containing red pigment which masks the chlorophyll. The colouring has its 

 relation to light. The greatest activity of photosynthesis is in light comple- 

 mentary to the colour of the plant. Ordinary green plants make special use of 

 the rays at the red end of the spectrum ; but for Red Algae rays further 

 along the spectrum are effective., and it is the rays towards the blue end of 

 the spectrum which penetrate into the depths of sea water. But all of the 

 Rhodophyceae are not red. It is significant that Lemanea, which is exposed 

 to ordinary sunlight in shallow fresh-water streams, is olive-green. 



In form the Red Algae are various, but never large. They include plants 

 which in form and colour are among the most beautiful, and therefore are 

 prized by collectors. They may consist merely of branched septate filaments : 

 or fronds, variously thickened and flattened, may be formed by matting 

 and webbing of many filaments together. Often they are fan-shaped and 



