222 t BOTANY PAET i 



The chemical composition of animals is essentially similar to that 

 of plants. The absorption of food in animals takes place by means 

 of the digestive system. The contrast is, however, not so great as 

 appears at first sight, for as a rule the food materials are converted 

 into a fluid condition before they are absorbed by the cells. 



II. The Nutrient Substances : their Absorption and their 

 Movement within the Plant 



The materials taken into a plant may be necessary, unnecessary, or 

 harmful. In any particular case this can only be decided experi- 

 mentally, for it would lead to erroneous conclusions to assume that 

 all substances constantly present in a plant are necessary. It has 

 indeed been found that only ten of the thirteen elements mentioned 

 above are indispensable. They enter the plant not as elements but 

 as compounds. We can distinguish as the three main groups of 

 nutrient substances (a) water, (b) salts dissolved in water, (c) gases. 



A plant cannot exist without a continual supply of nutrient sub- 

 stances. This is evident in the case of a growing plant in which the 

 increase in size of the body is at the cost of the material absorbed 

 from without. The fully-grown portions of the plant also require a 

 steady supply of new material, since their metabolism involves a 

 constant loss of substance. 



(a) Water 



All the chemical changes which take place in the metabolism of 

 the plant are carried out in WATERY SOLUTIONS. For this reason 



WATER IS AN INDISPENSABLE CONSTITUENT of the plant. All portions 



of the plant are permeated with water, and the protoplasm, the basis 

 of life, always contains 75 per cent or upwards of water. The plant 

 can only carry on its life fully when in this condition of saturation 

 with water. Any considerable diminution in the amount of water 

 either destroys the life permanently, or at least so greatly diminishes 

 the manifestations of life that they can no longer be observed. 



With the exception of some succulent plants wl^ch are uninjured by the loss 

 of nine-tenths of their water, plants as a rule have their activity impaired by the 

 loss of water in withering, and are killed by complete desiccation. It is always to be 

 regarded as due to some special provision or exceptional quality when entire plants, 

 or their reproductive bodies which have been dried, can be again brought to life 

 by a supply of water. Thus, for example, some epiphytic Ferns, some Algerian 

 species oflsoetes, and the Central American Selaginella lepidophylla, can withstand 

 droughts of many months' duration, and on the first rain again burst into life and 

 renew their growth. In like manner many Mosses, Liverworts, Lichens, and 

 Algae growing on bare rocks, tree-trunks, etc., seem able to sustain long seasons 

 of drought without injury. 



Seeds and spores after separation from the parent plant can as a rule endure 



