MOVEMENTS OF GROWTH. 



371 



The tension can finally be easily expressed in percentage of the 

 original circumference. Suitable material for determining the 

 existence of transverse tension is afforded by Helianthus stems, or 

 five to ten year old trunks or branches of species of Prunus, 

 Pyrus, or Salix. I have, e.g., investigated the tensions in trans- 

 verse slices, 5 mm. in thickness, from branches of Prunus insititia 

 and from a Salix. Circumference of discs 106 (Prunus), and 132 

 (Salix), respectively. Distances between cut surfaces, 4'5 and 

 6 mm. respectively. Tensions, 4'2 and 4'5 per cent, respectively. 



If in the manner described we determine the tension simul- 

 taneously at different levels in a stem structure, if, e.g., we take 

 slices from a stem of Helianthus annuus at the base, at the middle, 

 and at the upper end respectively, and then in each case isolate 

 the cortex, it will be found in general that the transverse tension 

 in the younger parts of the structure is relatively small, but that 

 it augments considerably in the older parts. 



High transverse tension in stem structures is associated with 

 the occurrence of rapid growth in thickness in them. When this is 

 proceeding the circumference of the central tissues (especially of the 

 wood) increases more rapidly than that of the peripheral tissues. 

 These consequently become stretched, and shorten on isolation. 

 But the resulting wood is by no means always of the same circum- 

 ference ; it may indeed vary considerably in size, which is easily 

 explained when we consider that the quantity of water of imbibi- 

 tion in the wood at any time exerts 

 a marked influence on its volume. 

 Increase in the quantity of water in 

 the wood must consequently result in 

 increased tension in the stem. If, e.g., 

 we cut slices from a branch of Prunus 

 insititia, determine the tension in 

 some of the slices at once, while the 

 others are first placed in water for 

 twenty-four hours, it is found that the 

 tension is greater in the latter than 

 in the former. 



I made with Prunus, using slices about * ~ kolafed a 



laid round the wood. 



15 mm. thick, and about 100 mm. in 



circumference, the transverse tension increased from 4'5 to 5'5 

 per cent, when the discs were left in water for twenty-four hours. 

 As regards the relations between the tension and other external 



FIG. 119. Transverse slice of a 

 In experiments which bough of Prunus. The coitex has 

 first been isolated and then again 



