496 BOTANY 



Fresh-water Algae. Bodies of fresh water are usually subject to 

 greater or less fluctuations of level, and in the cooler parts of the 

 world, to great extremes of temperature. It follows, therefore, that 

 plants living in fresh water must be able to endure a wide range of 

 temperature, and in case of shallow bodies of water, they must be 

 able also to withstand complete desiccation. It is very easily ascer- 

 tained that fresh-water Algae, as a rule, can endure much greater 

 changes of temperature without injury than can most marine Algae. 

 In special cases, such as Algae living in hot springs, they can some- 

 times endure a temperature approaching the boiling point, while 

 many Algae may be frozen into ice without injury. 



Protection against Drought. As most fresh-water Algae may be 

 exposed to drying up, we find that almost all of them develop some 

 means of resisting this. In many low forms like Oscillatoria, Pleu- 

 rococcus, etc., the vegetative cells may become completely dried 

 without suffering injury and will remain dormant for an indefinite 

 period, reviving very promptly when again supplied with water. 



Resting-spores. In a large number of Algae, however, special 

 cells are developed which are capable of surviving both drought 

 and cold, which destroy the vegetative cells. These resting-spores 

 may be formed by a transformation of a vegetative cell, as in Ana- 

 baena, or Kivularia, or they may be the product of fertilization, as in 

 the spores of Spirogyra or (Edogonium. 



Marine Algae. When we compare the conditions in the sea with 

 those of fresh water, it is evident that they are far more constant, 

 and the marine plants are, as a rule, much more sensitive to changes 

 especially in temperature, than are fresh-water forms, this being 

 especially true of the forms from deeper water and those of the open 

 ocean; and these forms are quickly killed by a change to warmer 

 water. 



The development of special pigments in the Bed and Brown Algae 

 is associated with the modification of the light rays which are con- 

 cerned in photosynthesis, and is characteristic of most of the marine 

 forms. 



While Seaweeds are never exposed to complete drying up for long 

 periods, many of them, like the Rockweeds and other forms growing 

 between tide-marks, are periodically exposed to the air at low tide. 

 It is found that resting-spores, such as those of the majority of fresh- 

 water Algae, are absent in most Seaweeds, and that the reproductive 

 cells, as a rule, germinate at once. In the case of some northern 

 forms, the plants die down in winter, but the basal part remains 

 alive, sending up new shoots each season. Protection against tempo- 

 rary drying is provided for in most Algae which are exposed at low 

 tide. Such forms are often of a marked gelatinous consistence, the 

 cell-walls being often highly mucilaginous, so that they retain water 



