726 MECHANICAL LAWS OF GROWTH, 



sufficiently powerfully to abstract it from the surroundinf^ tissues which must evi- 

 dently contain a much greater quantity of dissolved substances \ 



It is now clear from the observations which have been described, why portions 

 of shoots cut lengthwise in half or in four and placed in water curve outward to 

 such a remarkable extent ; and why a curvature, which may be small but continues 

 to increase for some time, takes place when such pieces are placed in a closed glass 

 tube in dry air. 



(2) Transvei'se tension caused by stihsequcnt thickening of the wood. It has 

 already been shown that transverse tensions also arise during growth caused by the 

 longitudinal tension, a more exact knowledge of which is still a desideratum. With 

 the commencement of the increase in thickness of the stem caused by the cambium- 

 ring, a new cause of tension arises, acting in both a radial and peripheral direction ; 

 and this transverse tension generally continues as long as the cambium-ring re- 

 mains active. The layers of tissue formed from the cambium-ring have at first 

 a tendency to expand in the tangential direction to an extent greater than the 

 space between the epidermis and the primary cortex permits. These outer tissues 

 therefore become stretched in the peripheral direction ; and, since they are elastic 

 and have a tendency to contract, they exert a pressure in the radial direction on the 

 cambium and the tissue formed from it, viz. the wood and the layers of secondary 

 cortex. It happens however also that the rings of wood produced on the inside 

 of the cambium grow more strongly in the tangential direction than the phloem 

 produced on the outside, which is therefore passively distended. A tension is hence 

 set up in the transverse diameter of the stem during its increase in thickness of 

 such a kind that each layer is stretched peripherally on its outside and compressed 

 radially on its inside ; in other words, is in a state of negative tension on its outside, 

 of positive tension on its inside. If the separate layers of a transverse segment 

 — epidermis, primary cortex, secondary cortex (phloem) and xylem — are separated, 

 and their peripheral length compared, we get the following expression for the 

 transverse tension : — 



E < C < Ph < X. 



As the increase in thickness proceeds the transverse tension increases, as is shown 

 by Kraus's very complete experiments ; i. e. if the rings of tissue in a transverse 

 segment of the stem or in a woody branch are separated from one another, by 

 dividing it longitudinally and then separating the rings, they contract the more the 

 nearer they lie to the circumference, and the contraction is the more considerable, 

 compared with the original circumference of the whole, the older the original segment. 

 The traction upon the cells of the epidermis and of the primary cortex caused by 

 the transverse tension is easily observed by the microscope in the transverse segment, 

 if young internodes of plants which increase rapidly in thickness, as Helianthus, 

 Ricinus, or Ribes, are compared with those which have already been forming wood 

 for some weeks or months. The form of the cells shows that they have been 

 strained in the peripheral (see Fig. 56, p. 69), and have in consequence grown 



^ I must content myself here with this preliminary sketch, which I shall carry out more in 

 detail in the Proceedings of ihe Wiirzburg Botanical Institute. Absorbent root-hairs and cortical 

 cells behave in the same manner as the pith. 



