204 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



of manures. In a general way we know that nitrogenous manure 

 tends to produce luxuriant vegetative growth, and that if applied in 

 excess it will encourage the development of qualities that may cause 

 many kinds of plants to fall an easy prey to the onset of disease. 

 Other manures, e.g. potash and phosphatic, may, but do not always, 

 encourage a storage of food reserves, and so indirectly bring about 

 fruitfulness and early maturity of the crop. It is unwise, if indeed it 

 is not impracticable, to generalize very widely on the effect of this or 

 that manure. Not only has its influence on the soil tilth, and its effects 

 on the other soil constituents, to be taken into account — and at present 

 we do not know enough about this — but there is also the chemical 

 idiosyncrasy of the plant to be reckoned with. By the chemical 

 idiosyncrasy I mean that which is often described as its specific nature. 

 But I prefer the former term, inasmuch as I believe it to express more 

 precisely what it is that we are really concerned with in distinguishing 

 one plant or group of plants from another. All the evidence at our 

 command which is worth anything points to the conclusion that the 

 essential features in which one species differs from another are just in 

 chemical properties, and that it is this immensely important fact 

 that lies behind, and is responsible for, the existence of those outward 

 characters to which we naturally have recourse in distinguishing 

 varieties, species, or genera of plants. I need do no more in this 

 connexion than remind you how often nearly related plants are 

 characterized by the common production of identical or at least of 

 nearly related chemical compounds. 



But in considering the plant from the point of view of water supply 

 in connexion with its environment, the physical structure and pro- 

 perties of soils must not be lost sight of. The work of recent years 

 has very clearly shown that the percentage content of water in a given 

 weight of soil may afford very misleading data as to its suitability 

 for supporting vegetation. In other words, the water is held in very 

 different degrees of fastness by soils of different character. A soil, if 

 it be of a clay or peaty nature and containing, say, 30 per cent, of 

 water, may be far less " wet " from the plant's point of view than a 

 sandy loam containing only 15 per cent. Physical experiments have 

 been devised to measure the availability of water in different soils, 

 and under carefully regulated experimental conditions the ratio 

 between the water extracted by a given centrifugal force from differ- 

 ent types of soil is found to accord very closely with that required 

 by plants when growing on those soils. Exact correspondence is often 

 disturbed in the open, where the conditions cannot so easily be 

 controlled, but we are aware of most of the sources of divergence 

 between theoretical and actually observed results, and we can to 

 some extent control the matter, thus enabling the plant to utilize 

 the soil water in a degree approximate to its theoretical value. The 

 soil consists of larger or smaller particles, the surfaces of which are 

 covered by a film of water. When water is withdrawn from a total 

 mass of soil these films become very thin, and the surface forces 



