332 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



elaborated sap, the true plant food of which the crude sap arising from 

 the roots only forms part of the raw material. 



The outer system of tubes referred to above is entirely occupied with 

 the transport of this material. This is easily seen to be so when a tree 

 receives an injury and the outer bark is removed. In time new bark will 

 be formed from above, thus proving that it is the downward and outer 

 flow of sap which is the material from which the bark is formed. 



It may be asked what determines the flow of this formative sap in the 

 direction required to roots, bark, or fruits ? To understand this we must 

 briefly consider the force of osmosis. 



If in a basin of water we float a porous cup of unglazed earthenware 

 or of parchment, containing a strong solution of salt, in a few hours it 

 will be found that some of the salt has filtered through into the water 

 surrounding the cup. This transfusion is called osmosis, and it will 

 continue until the solutions in the floating cup and the water in the 

 basin are of exactly the same strength. Therefore a steady flow of the 

 chemical salt is produced in one direction. It is precisely the same 

 process that decides where in the plant the sap shall flow to. For 

 instance, while the leaves are actively producing sugar which is conveyed 

 to the root, the roots may be storing it as starch. As each grain of starch 

 is formed in the root so much sugar is consumed, and the sap is therefore 

 less rich in sugar than the sap in cells above. A steady flow of sugar 

 is therefore maintained just so long as the transforming process in the 

 roots continues. 



This is a brief outline of the translocation of food-material formed 

 in the leaves and carried to the stems or roots. 



We may now consider how this supply is drawn upon for the 

 nourishment of fresh growths. The most convenient example of this is 

 the development of the bud. The buds are developed in the axils of the 

 leaves, and an examination of a strong shoot in winter will reveal 

 a certain difference between the buds situated on different parts of the 

 shoot. The buds at the base will be seen to be very small, while those 

 midway up the shoot are conspicuous and are covered with a downy felt. 

 The latter are fruit buds. Higher up the shoot the buds again decrease 

 in size, except perhaps the terminal bud of the shoot, which may be 

 a fruit bud again. The shoot can therefore be divided into three distinct 

 parts : dormant buds at base, fruit buds at the centre, and above these 

 the wood buds, viz. those which will produce only fresh branches and no 

 fruit. It may be asked : What is the cause of the difference of these buds ? 

 It is entirely a question of development. Given a sufficient food-supply all 

 buds will develop into fruit buds, but a deficiency results in either dormant 

 or growth buds only. Bearing in mind what has been said about the 

 importance of light, it will be seen that this factor alone will account for 

 the whole difference of the buds shown on the shoot. The dormant buds 

 at the base were nourished by leaves which were to a certain extent shaded 

 by those above and therefore not so capable of forming food-products. 

 The leaves above these are the most favourably situated and are produced 

 at a period of maximum sunshine, midsummer, and their activity is 

 therefore the greatest. The leaves higher still are borne on the second or 

 autumn growth, during which weather conditions are not so favourable 



