94 DISEASE IN PLANTS. 



the right quantities of water which dissolve just 

 the proper proportions of all the essential mineral 

 salts, but nothing poisonous, while the soil itself 

 has a texture such that the roots and root-hairs 

 can extend and do their utmost in absorbing. 



The leaves above are exposed to just the right 

 intensity of light, in air which is not too dry, 

 and is of suitable temperature and composition, 

 containing no poisonous exhalations, etc.; and as 

 the foliage is gently moved by the breeze, it 

 manufactures carbohydrates at the optimum rate 

 in the chlorophyll, and the so-called " elaborated 

 sap " containing the dissolved organic food- 

 supplies is prepared in the tissues in maximum 

 quantities and of just the right degrees of con- 

 centration and quality for use in the buds, stem, 

 roots, etc., for which it is destined as they draw 

 on the supplies. 



Between these assimilating organs, the leaves, 

 and the absorbing roots, we have in the stem the 

 wood, with its vessels adapted in quantity and 

 calibre to convey the water containing dissolved 

 salts from the absorbing roots to the leaves (to say 

 nothing of other parts) and, separated from this 

 wood by the cambium, we find the sieve-tubes and 

 cortical tissues in suitable quantity conveying the 

 " elaborated sap " the solutions of organic food- 

 materials from the leaves down to the roots, up to 

 the buds, and elsewhere. Joining these cortical 

 and wood tissues are adapted series of medullary 

 rays which, apart from other connections, bring 

 about the necessary interchanges of water and 



