XI. THE PLANT 221 



of the rays of the rising sun, and it has been alleged that such rays 

 possess some occult beneficial influence upon that growth. 1 



The writer feels fully convinced that the injury effected upon 

 plants by being deprived of the early rays of the sun, arises chiefly 

 from their being hurried through the temperature at which maximum 

 growth is possible. The abundance of sunshine which one is apt to- 

 consider so valuable an aid to plant growth in sunny South Africa 

 would thus seem to be in excess of their requirements, and probably 

 the country would be more productive if there were less of it. 



Transpiration through the leaves is also very active in this dry 

 climate, and direct experiment by the writer in 1904, showed that a. 

 potato plant exhaled 467 grammes of water for each gramme of solid 

 matter formed. This was in ordinary soil without manure. Another 

 experiment showed that only 284 grammes of water for each gramme 

 of solid matter were given off from a potato plant grown on soil 

 manured with 800 Ib. basic slag and 100 Ib. potassium nitrate per acre. 



Now a plant exhaling moisture rapidly during the hot part of the 

 day, when the whole plant and its roots have been heated by the sun, 

 suddenly has its rate of transpiration checked by the cooling of its- 

 leaves after sunset. Its roots, however, are still active in absorption 

 of water from the hot soil, and undoubtedly disturbances in the pres- 

 sure of the sap, often of an injurious character, must be set up. Such 

 disturbances may give rise to rupture of some of the cells of the plant, 

 and is probably the cause of a " disease " which has long puzzled 

 pathologists viz., <: Bitter pit " in apples. 



During germination oxygen is actually absorbed in considerable 

 quantity, carbon dioxide being exhaled. 



The process of oxidation produces heat, and a considerable rise of 

 temperature may be observed when many seeds germinate together 

 (e.g., in malting barley). During this slow combustion, the fats and 

 carbohydrates are consumed, but the proteid matters remain undi- 

 minished in quantity. 



In germination, various enzymes present in the seed commence to> 

 act (diastase, which has the power of converting starch into maltose 

 and dextrose, is a typical one), whereby the reserve materials in the 

 seed are converted into soluble substances, capable of transportation in 

 the sap. The embryo increases rapidly in size, sending upwards a, 

 plumule, which will eventually produce the stem, and downwards a, 

 radicle, destined to form the root. The direction of growth of the 

 plumule and radicle is determined by gravitation, for if seeds be ger- 

 minated while subjected to centrifugal force (say on a rotating wheel),, 

 the plumule grows towards and the radicle away from the centre of 

 rotation. As soon as the plumule reaches the surface of the soil and 

 becomes exposed to light, chlorophyll is produced in it and assimila- 

 tion commences. The radicle, too, develops root-hairs and becomes, 

 capable of taking in plant food from the water of the soil. 



Before describing the chemical changes which occur in plants, it 

 may render the matter clearer if a few terms which are used in phy- 

 siology be explained. 



1 Kawson, Transvaal Agric. Journal, April and July, 1906. 



