i86 A TEXTBOOK OF THEORETICAL BOTANY 



is greatest during the first few hours of exposure. Algae growing high up 

 lose water more slowly than those lower down, but the total loss is greater 

 and may reach nearly 70 per cent, of the original weight. No doubt this 

 loss falls principally on the intercellular mucilage and to some extent on the 

 material of the thick cell walls, so that the protoplasm is protected from 

 destruction, but when dry, assimilation is completely inhibited by the slowness 

 of gaseous diffusion through the dry tissues and respiration is extremely 

 slow. Thus growth is much less in the high zones than in the lower zones. 

 Chemical composition also plays a part in protection, for the Algae of the 

 highest zones have the highest fat content, and those of the lowest zones 

 the highest content of carbohydrate, the two being inversely related in the 

 intermediate zones. It has also been found that Fuctis tolerates much greater 

 changes of temperature than Algae of the lower levels, e.g.,Laminarta, which 

 appears to be correlated with regular exposure of the former to sunshine 

 at low tide. 



In rock pools conditions are quite different. Here there is no desiccation 

 and assimilation can continue at low tide. In small pools with a rich algal 

 flora, however, the Carbon dioxide content of the water is soon exhausted. 

 This leads to dissociation of the bicarbonates of Calcium and Magnesium 

 present in solution. The carbonates of these metals are only slightly soluble, 

 but they suffice to give the water a strongly alkaline reaction, which increases 

 steadily with the length of the inter-tidal period. In the highest pools only 

 a few seaweeds can withstand the effects of this high alkalinity, and such 

 pools have a very limited flora, mostly of Chlorophyceae. 



Perennation of Seaweeds 



As no resting stage is known in the life-history of any seaweed, it was 

 for long a problem how they passed through the winter season. Many 

 seaweeds are invisible at this time, while others disappear during periods 

 at other times of the year. The solution was discovered by Sauvageau, who 

 found that at these times the plants are represented by very small, almost 

 microscopic thalli, which are prostrate and discoid or filamentous in structure. 

 Numerous Algae have been found to show this alternation, among them being 

 Ectocarpus and Cutleria. 



These microthalli are called the adelophycean stage. They seem 

 usually, perhaps always, to be monoploid and to reproduce the macroscopic 

 plant sexually, by gametes formed in gametangia. If this is the case they form 

 an interesting comparison with the microscopic gametophytes oi Laminaria. 



The separation of prostrate and upright generations among these Algae 

 may be regarded as a further development of the heterotrichous habit 

 shown by many Algae in all groups, namely the distinction between prostrate 

 and upright portions of the thallus. Fritsch has laid great stress upon the 

 widespread occurrence of this habit among the Algae as foreshadowing and 

 perhaps actually originating the distinction between prostrate or reduced 

 gametophytes and upright sporophytes which is characteristic of the lower 

 land plants. 



