2] THE VARIOUS GROUPS OF PLANTS 39 



in the different genera or even species, ranging from filamentous 

 (very rarely unicellular) to densely branched and coral-like (owing 

 to encrustation with ' lime '), and from a flat blackish disk to a ribbon- 

 shaped or widely expanded ' frond '. Attachment to the substratum 

 is by disk-like holdfasts or special filaments ; or the whole plant 

 body may form a close investment. Many Red Algae are very 

 intricate and beautiful in form : a range of examples is shown in 

 Fig. 9. The internal structure is peculiar and relatively complex, 

 though none of the Red Algae approaches in size the larger Brown 

 Algae. 



The nutrition of the Red Algae is much like that of the Phaeo- 

 phyceae, except that many of the chemical products of photosynthesis 

 and subsequent metabolism are different ; these include the food- 

 storage materials, of which a unique starch-like substance is the 

 chief. The reproduction is remarkable in lacking any self-propelled, 

 flagellate stage. Sexual reproduction is effected by fertilization in 

 situ of a large and fixed female cell by a small male one or its dis- 

 charged contents — in either case carried along, aimlessly as it were, 

 by an impinging water current. Instead of resulting in the formation 

 of a new, ' daughter ' plant, fertilization leads to further development 

 which results in the formation of special asexual spores called 

 carpospores. These in the simpler Red Algae germinate to produce 

 sexual plants ; but in the vast majority of types they give rise instead 

 to asexual plants producing another kind of asexual spores, called 

 tetraspores, which in their turn germinate to produce sexual plants. 

 In such cases there is a regular alternation of a sexual generation 

 with two asexual ones of which the second is on a separate plant. 



The Red Algae are very widespread. Not only do they occur in 

 fair numbers in the habitats occupied by Brown Algae — with which 

 they are commonly interspersed even to the extent of frequently 

 growing superficially on their bodies, as epiphytes — but there are 

 also a number and range of forms inhabiting cool streams and other 

 freshwater habitats. Many of the marine types tend to grow in 

 deeper water than the Brown or Green Algae, being supposedly 

 adapted through their special pigments to photosynthesize far under 

 the surface of the water by absorbing the shorter wave-lengths of 

 light which penetrate to relatively great depths. Red Algae also 

 tend to be more numerous in warm than in cold seas, although not 

 a few occur well north in the Arctic. At their best they may dominate 

 the deeper layers, especially, of marine coastal vegetation. Of the 

 Red Algae, again, the carbohydrates and carbohydrate derivatives 



