EFFECT OF MECHANICAL AGENCIES ON STEREOME 195 



from the ground, the upper parts of the steins and branches being left 

 in their natural state. The trees were planted so as to be freely 

 exposed to the action of the wind. " In the succeeding summer, much 

 new wood accumulated in the parts which were kept in motion by the 

 wind ; but the lower parts of the stems and roots increased very little 

 in size." In the following winter one of these trees was fixed so that it 

 could only move towards the north and south. " Thus circumstanced, 

 the diameter of the tree, from north to south, in that part of its stem 

 which was most exercised by the wind, exceeded that in the opposite 

 direction [i.e. from east to west], in the following autumn, in the 

 proportion of thirteen to 



eleven." There can be no Sl 



doubt that an unequal 

 production of wood-fibres 

 was partly responsible 

 for the greater increase 

 in thickness on the two 

 sides that were subjected 

 to the more severe me- 

 chanical stress. 



Recently, observations 

 of a similar nature have 

 for the first time been 

 carried out upon ten- 

 drils. It has long 



i i ,i , i -i Tendrils of Cyclanthera explodens in T.S. For explanation see text. 



been known, that tendrils y 



which fail to grasp a support remain much thinner than those which 

 become attached in accordance with the function for which they are 

 designed ; as a rule, also, unattached tendrils soon die, dry up and fall 

 off. The adjoining, figure (Fig. 70) depicts transverse sections across 

 the coiled regions of two equally old tendrils of Cyclanthera explodens, 

 one of which (A x ) has grasped a support, while the other (Bj) has re- 

 mained unattached. The former shows, on its inner side, a well-developed 

 ribbon-shaped tension-flange composed of two or three layers of very 

 thick-walled bast-fibres (A) ; further, each of the two vascular bundles 

 which run near the margins of the outer side is provided with a 

 strong sheath of bast, comprising from seven to ten cells in transverse 

 section. In the unattached tendril, on the other hand, the develop- 

 ment of the fibres in the stereome-ribbon has been arrested at an 

 early stage ; except in isolated cases the cells are only slightly 

 thickened in a collenchymatous manner, and are soft and consequently 

 often distorted (B 2 ). The two vascular strands on the outer side are 

 entirely devoid of fibrous sheaths. 





t 



Pig. 70. 



