454 GENERAL MORPHOLOGY OF PLANTS 



withdrawn from the cells into the intercellular spaces where the 

 ice crystals are gradually formed. This is in reality a protection 

 to the protoplasm, since it becomes " drier " and thus more re- 

 sistant to cold. When the plants " thaw " out, if the protoplasm 

 has not been killed by the cold, this water may be absorbed by 

 the protoplasm again, and the plant is not injured. Freezing, 

 then, to a certain extent has the same effect on the protoplasm 

 as the loss of too much water by transpiration. The bud-scales 

 thus prevent the loss of too much water from the tells of the deli- 

 cate tissues of the growing point of shoots in winter, although in 

 many cases freezing in these tissues takes place. Third. In 

 other cases plants are killed by the actual freezing of the proto- 

 plasm, though there are some plants which are not killed by 

 freezing of the water in the protoplasm. 



618. Climatic factors. These factors are operative over 

 very wide areas. There are two climatic factors: rainfall or 

 atmospheric moisture, and temperature. A very low annual 

 rainfall in warm or tropical countries causes a desert ; an abun- 

 dance of rain permits the growth of forests ; extreme cold prevents 

 the growth of forests and gives us the low vegetation of arctic and 

 alpine regions. 



619. Biotic, or life, factors.* These are animals which act 

 favorably in pollination, seed distribution, or unfavorably in 

 destroying or injuring plants, and man himself is one of the great 

 agencies in checking the growth of some plants while favoring the 

 growth of others. Plants also react on themselves in a multitude 

 of ways for good or evil. Some are parasites on others ; some in 

 symbiosis (see paragraph 207) are aided in obtaining food; shade 

 plants are protected by those which overtop them ; mushrooms 



* These biotic factors (plants and animals) in many cases do not exert a 

 personal or direct influence themselves on the vegetation concerned. They 

 often merely introduce physical factors, as shade, etc. But in some of the 

 more intimate relations of plants in symbiosis, parasitism (one form of 

 symbiosis), etc., the living organism as a factor is more evident, but it should 

 be understood that this factor is not in the nature of vitalism in the sense 

 of the old vitalistic theory. 



