382 ECOLOGY 



parts always covered by sea-water are numerous red 

 seaweeds. All these are Algae, a group of plants of much 

 more lowly organization than the flowering plants, and 

 differing widely from the latter in not having true roots, 

 stems, or leaves, and producing no seeds. 



Salt-marshes. Between the tide-marks is a belt destitute 

 of vegetation ; here the ground is regularly under the influ- 

 ence of the waves. If we listen to the roar of sand and 

 pebbles as the waves roll inwards and retreat, and watch 

 the constant movement of the surface, we can realize how 

 difficult it is for plants to secure a root-hold in such unstable 

 ground. In areas where the ground is bare for longer 

 intervals, as along the shores of sheltered bays, in estuaries, 

 or the banks of tidal rivers, land-plants establish themselves, 

 The muddy soil which accumulates in such localities 

 provides a peculiar habitat for plants. It is badly aerated, 

 liable to be covered by sea-water at very high tides, and 

 there is often much salt in the ground-water. On the other 

 hand, during exposed intervals, much evaporation may take 

 place, leading to a concentration of salt in the soil. Or the 

 reverse is possible ; during heavy rains much salt may be 

 washed out of the soil. Under such conditions it is not 

 surprising to find the ground occupied by a peculiar type of 

 vegetation. 



A habitat of this kind is known as a salt-marsh (Fig. 245). 

 The plants growing here have usually fleshy leaves covered 

 with a waxy bloom or grey hairs ; some develop short 

 hairs which break off and form a mealy covering over the 

 surface. Most salt-marsh plants have a reduced transpiring 

 surface, and store water in their fleshy tissues. They thus 

 possess many of the characteristics of xerophytes ; some of 

 them, however, e. g. the Glassworts (Salicornia spp.) and 

 Sea Aster (Aster Tripolium) , transpire freely, and are even 

 able to absorb water by their green surface. Salt-marsh 

 plants are known as halophytes. 



