NUTRITION. 



-H 



portions or amounts as may be required are 

 utilized or deposited, whilst what is not wanted 

 is evaporated in the form of vapour through 

 the stomata, or, in some cases, in a liquid state 

 through certain openings called " water-pores ". 



Evaporation, then, is one function of the leaf, 

 and we have seen how its amount is regulated 

 by the structure of the plant itself, and by 

 the conditions of temperature and moisture 

 under which it is placed, how the structure 

 of the plant is adapted to promote or to check 

 evaporation as the case may be. The great 

 diversity in the surface of the plant, the nature 

 and number of its hairs, &c, have reference to 

 the regulation of the passage in or out of gases 

 or liquids. 



Respiration or breathing is another most im- 

 portant function carried on in the leaves and in 

 all the parts of plants. This interchange of gases 

 goes on in the protoplasm of every living cell. 

 As, however, the arrangements favouring respi- 

 ration are most conspicuous in the case of leaves, 

 it is convenient to speak of them in this place. 



The process of respiration consists mainly in 

 the absorption of atmospheric air, and in the 

 exhalation of carbon dioxide gas. It is a pro- 

 cess which, like the respiration of animals, is 

 going on continually in darkness as under ex- 

 posure to light; heightened and accelerated by 

 some conditions, it is diminished or retarded by 

 others. In animals the process is regular and 

 rhythmical, but no such periodicity is observ- 

 able in plants. The chemical results of this 

 interchange of gases are of great importance to 

 the plant, and should be carefully studied in 

 treatises dealing with the chemistry of vegeta- 

 tion. 



Assimilation, or the production and utilization 

 of nutrient materials, is a function of the leaf 

 that is only carried on in daylight. Chlorophyll, 

 or green matter, is formed within the protoplasm 

 under the agency of light, and in the chlorophyll 

 liquid sugar and solid starch are formed in direct 

 consequence of exposure to the light, an ex- 

 posure always attended with an exhalation, not 

 only of watery vapour, but of oxygen gas also. 



Here again the chemical processes involved 

 are numerous and complex, and the results 

 equally so. The infinite variety in form and 

 position of the leaf have reference, as has been 

 said, to the protection of the leaf from injurious 

 agencies of all kinds, and to the promotion of 

 the proper work of the leaf. A detailed know- 

 ledge of these forms and processes must be left 

 to chemists and physiologists; it is enough for 

 the young gardener to know that the leaves are 



among the most essential parts of a plant, that 

 their principal offices are as just explained, and 

 that it is his duty to place the plant under 

 such conditions as may be most favourable to 

 their action. Thus, he will have to apportion 

 the amount of air and water supplied, to regulate 

 the temperature of his houses, and the intensity 

 of the light, and to adjust the quantity of the 

 one to the amount and intensity of the others 

 according to circumstances, according to the 

 structure of the plant, and according to the 

 object he has in view. 



It is obvious that practice and experience are 

 indispensable in such a case, but the experience 

 may be gained more quickly, and utilized more 

 completely, if the gardener has an adequate 

 general knowledge of the principles upon which 

 his art is based. [m. t. m.] 



CHAPTER VI. 



NUTRITION. 



Conditions of Plant Life — The Soil as a Source 

 of Food and Air— The Air as a Source of 

 Food — The Action of Light— Chlorophyll — The 

 Movements of Liquids in Plants — The Sap. 



In the preceding paragraphs attention has 

 been drawn to those parts most particularly 

 concerned in the nutrition of the plant. Their 

 structure and functions have also been indicated, 

 so that we are now in a position to deal with 

 the nutrition of plants as a whole. In the first 

 place, it is obvious that the food of plants, or 

 those things which are capable of being con- 

 verted into food, are to be sought in the soil 

 and in the air. A certain degree of heat, neither 

 too little nor too great, but varying within wide 

 limits according to the nature of the plant, its 

 particular requirements at different periods, and 

 the locality in which it grows, is also essential. 

 The soil in winter and spring is always warmer 

 than the air, hence the importance of "bottom 

 heat" in plant culture. Light is indispensable 

 in some, but not in all cases. Those plants that 

 can live upon others as parasites, or upon ready- 

 made food supplied by decaying vegetable or 

 animal substances, do not themselves need to 

 be exposed to the light. Of such nature are 

 mushrooms. 



The Soil as a Source of Food, Water, 

 and Air. 



The soil is composed of mineral or inorganic 

 ingredients derived from the weathering and 



