ADJUSTMENT TO TEMPERATURE 99 



of the migrants. Seeds, fruits, etc., will move equally well in 

 all directions, provided the proper agents are at hand for carry- 

 ing them. Considering temperature alone, their ecesis is more cer- 

 tain if they move to the east or west than if they move south- 

 ward or northward. Generally, also, the chances of establish- 

 ment are greater southward than they are northward. In the 

 first case, migration to the east or west does not essentially change 

 the relation of the plant to temperature. Migration southward 

 means that the plant must accustom itself to higher temperatures 

 as well as to a greater annual sum. While this is taking place, 

 the plant may be at a disadvantage in competition with other 

 plants already well established. Plants that migrate north- 

 ward, in addition to a corresponding adjustment to lower tem- 

 peratures and a lower sum, run an increasing risk of encountering 

 a fatal minimum. This risk is greatly increased by the fact that 

 southern plants require a longer period for their life cycle than 

 northern ones, and in a northern habitat are often unable to reach 

 maturity before the regular appearance of fatal frosts. So far 

 as the controlling influence of temperature is concerned, plants 

 spread readily to the east and west, less easily to the southward, 

 and least easily to the north. The same fundamental rule applies 

 to mountains, the increasing cold upward making ecesis more r* 

 uncertain than it is downward. The grouping of species, which 

 forms vegetation, is in accordance with this fact. In consequence, 

 vegetation exhibits zones extending east and west upon conti- 

 nents, and lengthwise along mountain ranges, which are due 

 largely to temperature. The disturbing influence of other factors 

 and the discussion of vegetation zones is considered under Zonation. 

 121. Digestion. The complex materials, starch, cellulose, 

 oils, and jiroteids, stored as reserve food in various parts of the 

 plant, must be dissolved or othenvise chemically changed before 

 they can be used. This process is digestion. It is carried on by 

 the protoplasm or by certain peculiar products of it, called enzymes 

 or ferments. In chlorophyllous plants, digestion takes place 

 within the living cells, except in certain insectivorous forms where 

 the secretion is poured out upon the surface, as in the pitcher- 

 plant and Venus' fly-trap. Fungi, on the contrary, regularly 

 carry on digestion outside of their own cells, either digesting the 

 food before it is absorbed, as in many saprophytes, or making use 

 of the enzyme to permit their entrance into the tissues of host- 



