FOOD AND WATER MOVEMENT. 



269 



OxrotM 



These food materials are, by tlie life processes of the active cells, changed in chemical 

 composition as need be, from sugar, which is soluble, into starch, which is insoluble, and back 

 into sugar, and combined with nitrogenous substances to make the cell-forming material, proto- 

 plasm (fig. 27). 



In the fall, when the leaves cease to elaborate food, both the upward and the downward 

 movement, more or less simultaneously, come to rest (the surplus of food materials, as starch, and 

 sometimes as sugar, being stoied for the winter in certain cell tissues), to begin again simultane- 

 ously when m spring the temperature is high enough to reawaken activity, when the stored food of 

 last year is dissolved and started on its voyage. The 

 exact manner in which this movement of water upward 

 and food materials downward takes })lace, and the 

 forces at work, are not yet fuily understood, nor is 

 there absolute certainty as to the parts of the tree in 

 which the movement takes place. It appears, how- 

 ever, that while all the so called "sapwood" is capable 

 of conducting water (the heartwoocl is probably not), 

 the most active movement of both water and food 

 materials takes place in the cambium (the growing 

 cells immediately beneath the bark) and youngest 

 parts of the bark. 



The deductions from these processes important to 

 the planter are: That injury to the living bark or bast 

 means injury to g.rowth,if not destruction to life; that 

 during the period of vegetation transplanting can be 

 done only with gieat caution; that the best time to 

 move trees is in the fall, when the leaves have dropped 

 and the movement of water and food materials has 

 mostly ceased, or in spring, before the movement be- 

 gins again, the winter being objectionable only because 

 of the difficulty of working the soil and of keeping the 

 roots protected against frost. All things consideied, 

 spring planting, before activity in the tree has begun, 

 is the best, although it is not impossible to plant at 

 other times. 



In the making of protoplasm and the plant tissues 

 resulting from its growth, many chemical changes 

 occur within the plant, as a result of which not only 

 woody tissue, which may be considered the permanent 

 essential product of growth, but also many by-products 

 are formed. It not infrequently happens that what 

 has here been termed the by product is of greater com- 

 mercial importance than the wood itself. Thus among familiar woody species the India rubber 

 tree is only valued for its sap, the logwood of Central America for dyeing, the cinchona trees for 

 their alkaloid (quinine products), etc. Again, some of our most important timber trees yield also 

 useful by-products. The maple yields millions of pounds of excellent sugar, the longleaf pine is 

 the principal source of the resin and turpentine supply of the world, and the bark of hemlock and 

 of certain oaks furnish most of the tannin used in American tanneries. 



Fig 27 — Physiological importance of different parts ot 

 the trie, pathways ot watei and food materials. 

 (Schematic ) 



PROGRESS OF DEVELOPMENT. 



Like wheat or corn, the tree seeds require as conditions for sprouting sufficient moisture, 

 warmth, and air. The seeds, however, differ from grain in that most of the kinds lose their power 

 of germination easily; with few exceptions (locust, pine, spruce), they can not be kept for any 

 length of time. 



The hrst leaves formed often differ essentially in shape from those of the mature tree, which 

 may cause their being confounded with other plants, weeds, etc. 



