773 



that the growth in length of each branch of our trees is regulated by two 

 factors. 



The central tissue of the shoots, especially the pith, is tke elongating 

 factor^ the tissue which forces the shoot into the air. Its very considerable 

 striving to grow longer and to carry the surrounding tissue with it into the 

 air which becomes evident in an isolation from other tissues is modified and 

 retarded by the strain exercised by the very elastic peripheral tissue parts of 

 the bark body. These contract and become shorter if isolated. They 

 uniformly grow shorter even in their natural position on the tree in the night 

 because of a radial swelling resulting from the taking up of water-. 



Therefore, as the shoot grows, there develops a considerable longitudinal 

 tension due to the struggle of the elongating force of the surrounding tissues, 

 at times of the bark body, to contract both themselves and those surrounding 

 them. The result of this struggle is evidenced in the length of the pith cells 

 within one internode. Cell measurements have shown that the pith cells are 

 longer at first than they are later and that a very strong growth in breadth 

 is associated with their subsequent shortening. 



This increase in breadth is the result of the ultimate preponderance of 

 the peripheral strain. When the increase in length of the internode is 

 complete, the cross tension becomes great. 



It is easy to understand that other strains must occur after the length- 

 ening of a plant part is ended when one considers that the part of the trunk 

 which has already elongated now thickens permanently and that this thick- 

 ening depends upon the differentiation of the cambial cells, lying between 

 the bark and wood, into new wood and bark elements of the following year ; 

 the year old shoot forms new wood layers above those of the previous year ; 

 these new wood layers must make room for themselves under the girdle 

 formed by the bark and its outermost cork layers. This can be done only 

 by a distension of the bark mantle which, however, does not give way without 

 resistance. This resistance makes itself felt in pressure and thus, during 

 the period of the growth in thickness of a shoot, we find the tender tissue of 

 the cambium pressed on one side by the mature but still distending young 

 wood and on the other side by the constricting influence of the bark mantle, 

 which gives way only to very strong pressure. 



Under this double pressure, the elements of the wood are formed from 

 the cambium, that is, the elongated, thick-walled wood cells, poor in contents, 

 or finally entirely empty, as well as the ducts and duct-like cells. 



De Vries^ has now determined experimentally that the cells of the wood 

 become narrower (and the ducts fewer) the greater the bark pressure. He 

 increased the constricting effect of the bark mantle by putting on a firm 



1 According to Kraus (loc. cit., p. 141), Hales had already adopted the theory 

 expressed by Borelli in his book "de motu animalium" that "The young shoot grows 

 and elongates by the spread of the moisture in the spongy pith." 



2 Kraus, G., Uber die Vei'teilung und Bedeutung des Wassers bei Wachstums- 

 und Spannungsvorgangen in der Pflanze. Bot. Zeit. 1S77, p. 595. 



•■i Hugo de Vries, tJber den Einfluss des Rindendinickes auf den anatomischen 

 Bau de Holzes. Flora 1S75, No. 7, Sanio, Bot. Zeit. 1863, p. 393. 



