THE METABOLISM OF SEAWEEDS
The habitat of a seaweed differs greatly from that of the majority of land plants so that it is only natural to find that they have a number of physiological peculiarities.
When submerged they receive light that has been filtered through varying depths of sea water and which is, in consequence, deprived of most of the red and yellow rays which are those chiefly absorbed by chlorophyll. The view has been widely held that the red and brown pigments are functionally important in that they can absorb the blue and green rays of sunlight and so obtain photosynthetic energy for the plant when under water. Red Algae can grow down to depths of about 12 fathoms on our coasts and much deeper in clear seas, but in dull weather the maximum rate of photosynthesis appears to be at or near the surface and in bright weather about 2 fathoms down. This does not altogether bear out the adaptation idea, and suggests that the red colour is possibly only a protection for the chlorophyll against overbright sunlight, and that the seaweeds are simply shade plants with particularly sensitive chlorophyll. It is perhaps worth noting here that only chlorophyll a occurs in the Red and Brown Algae, the other form, chlorophyll f3, being absent. On the shore the dominant physiological factor is undoubtedly exposure, which includes both bright light and desiccation. The rate of drying
is greatest during the first few hours of exposure. Algae gro\ying 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. Ko 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 lo\yest zones the highest content of carbohydrate, the two being inversely related in the intermediate zones. It has also been found that Fucus tolerates much greater changes of temperature than Algae of the lower levels, e.g., Laminaria, 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 \yith a rich algal flora, however, the Carbon dioxide content of the \yater is soon exhausted. This leads to dissociation of the bicarbonates of Calcium and l\Iagnesium 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.
PERENKATION 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. l\Iany 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 of 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.
