196 MECHANICAL SYSTEM 



At this point reference may also be made to Treub's observations 

 upon the active increase in thickness which takes place in the irritable 

 climbing hooks of Artabotrys, Ancistrocladvs, Uncaria, etc., when these 

 organs take hold of a support. In such cases the primary object of 

 the secondary thickening seems to be the production of an increased 

 number of mechanical elements. As regards the stimulus which starts 

 the secondary growth, it is unknown whether the pressure of the 

 support upon the hook, or the longitudinal tension experienced by the 

 latter after fixation, plays the greater part. The same uncertainty 

 prevails in the case of ordinary tendrils. 



Interesting experimental results have been obtained by Worgitzky 

 with regard to the tensile strength of certain tendrils ; the subjoined 

 figures refer to two spirally coiled tendrils of Passijlora quadrangula/ris. 



Load which just T . . ,.. 



Load at the elastic suffices to straighten . \ . 



limit in grams. all the coils, in /' " .ff s 



grams. =^' 



Attached Tendril, - 40 500 600 



Unattached Tendril, 8 250 350 



In the first [German] edition of the present work the author drew 

 attention to the lack of any detailed experimental investigation of the 

 effects of artificial tension or compression upon the development of 

 the mechanical system. Since then quite a number of physiologists 

 have carried out researches of this nature, but for the most part 

 without arriving at any positive or unequivocal conclusions. The 

 most remarkable results are those obtained by Vochting in the course 

 of certain experiments upon potted plants of Curly Kale (Brassica 

 oleracea, var. bullata), which were placed in a horizontal position, with 

 weights attached to the distal end of the -stem. He found that in 

 these circumstances the secondary woody cylinder grew more actively, 

 and contained more mechanical elements, along the upper and lower 

 sides of the stem that is to say, in the regions of greatest tension 

 than it did on its flanks. In other experiments of Vochting's, pump- 

 kins were allowed to ripen, not, as is usually the case, while resting 

 upon the ground, but while hanging freely in the air by their stalks, 

 so that the latter were continuously subjected to considerable longi- 

 tudinal tension. Here again a general increase in the thickness of the 

 cell-walls was observed, not only in the case of the mechanical tissues 

 but also in that of the parenchymatous ground-tissue ; the secondary 

 wood was likewise found to contain an unusually large proportion of 

 mechanical elements. 



In subsequent researches upon this subject, allowance will have to 

 be made for the fact that a plant cannot be expected to accommodate 



