PLANTING. 17 



space of a fortnight, or three weeks, according to the age of the plant and 

 the state of the weather. A second ascent of the sap, and growth of shoots 

 now take place, but with diminished vigour ; unless from accident, dis- 

 ease, or unfavourable weather, the spring growth has been checked, and 

 the first flow of sap prevented from being exhausted in the production of 

 branches, leaves, and blossoms. It is worthy of remark, that those shoots 

 which form fruit, flower, or seed buds, have seldom if ever any second 

 growth ; but remain without increasing in length until the next spring. 

 The midsummer growth is almost always confined to thosejbranches which 

 carry wood buds only. After the second growth is completed, the effects 

 of the descending sap in the formation of new bark and wood is very 

 apparent in the healing up of wounded parts of the stem and branches, 

 which now proceeds with more activity than during any other period of 

 the year. Branches pruned off after the midsummer flow, seldom are 

 followed by shoots from the edges of the wounds caused by their removal, 

 which always happens, more or less, when pruning is performed on free 

 growing trees after the fall of the leaf, and before the full development of 

 the spring shoots and leaves : it is to be observed, however, that the repro- 

 duction of branches from the edges of a wound is greatly assisted by 

 leaving a portion of the branch or shoot, or its parent branch or stem, 

 but impeded when a branch is pruned off close to the stem. What was 

 before stated regarding the offices of the pith and medullary rays in 

 originating the buds of shoots and branches, will be confirmed by these facts. 

 Food of Plants. Those substances which the roots of plants take up 

 from the soil, and those which the leaves or green system of the plant 

 inhale or imbibe from atmospheric air are comprehended under the 

 name of the food of plants. This part of vegetable physiology has long 

 engaged the anxious inquiries of science, as well as of practice. The 

 question is one of much importance, inasmuch as a perfect knowledge of 

 what constitutes the food of plants generally, and individually, would with 

 unerring certainty point out the means of fertilizing soils, defective in any 

 respect for bringing to perfection the species of tree most desired ; would 

 indicate at the same time the most proper substances to be used with the 

 greatest advantage, the exact proportions in which they should be mixed, 

 the mode of applying them, and the best process of manual culture or 

 working the soil, for elaborating and preparing them for absorption by the 

 roots. Of late years great progress has been made in the investigation of 

 this part of vegetable physiology ; the labours of T. A. Knight and M. 

 Dutrochet are, in particular, highly valuable, but much still is required 

 before even an approximation to the solution of this important question can 

 be attained. The structure of the root shewed us that whatever kinds of 

 substances are conveyed or by it introduced into the plant, such substances 

 must be in a minute state of division, or dissolved in water. The analysis 

 of a soil demonstrates the soluble substances it contains. These have been 

 found to be chiefly vegetable extract, combined with smaller proportions of 

 a few of the neutral salts, as sulphates of potash and lime, muriates of 

 lime and soda, or common salt ; this last, in every instance of our own. 

 individual experience, is always in a larger proportion to the other saline 

 matters, and is never altogether wanting, as is the case sometimes with 

 the sulphates and muriates of lime. The vegetable extract, except as 

 regards its presence in poor clays and siliceous sands*, is always in a 



* The soils here alluded to, the results of whose chemical examinations have led to 

 the above conclusions, were of almost every kind or description to be met with in practice, 

 comprising the various degrees of fertility intermediate between the poorest sand and 

 the most tenacious clay. 



C 



