STEMS 



699 



The influence of external factors upon mechanical tissues. Mechan- 

 ical stimuli. An unattached tendril has less tensile strength than one 

 that is attached, the latter sometimes being two to five times stronger than 

 the former. Frequently this change in tensile strength is associated with 

 a structural change, more mechanical cells being developed or further 

 wall thickening appearing in those already present (figs. 1023-1026). In 

 some cases collenchyma 

 develops into bast upon 

 the attachment of the 

 tendril. It has been 

 claimed that growing 

 stems of Helleborus sub- 

 jected to tension develop 

 bast in regions where 

 ordinarily none is pres- 

 ent; furthermore, stems 

 subjected to tension for 

 three days have been 



V K 



found able to support a 



weight of 3500 grams, 



,.. , . FIGS. 1023-1026. Cross sections of tendrils of Cyclan- 



1 thera explodens: 1023, a diagrammatic cross section of 



of the experiment only a tendril that has not come in contact with a support; 



4OO grams Could be SUP- I02 4> a diagrammatic cross section of a tendril that has 



, rp, ... come in contact with a support; note the larger size of the 



in tendril, the increased development of the chief mechanical 



results, though fre- strand (m), and the development of accessory mechanical 



QUently tested have not stran ds (/'); v, vascular tracts; 1025, a portion of the 



\\ fi A 'th mechanical strand of 1023, highly magnified, showing 



M r collenchymatic wall thickening; 1026. a portion of the 



in Helleborus or in Other chief mechanical strand of 1024, similarly magnified; 



plants though it ap- note tne highly thickened walls, the elements being bast 



. . rather than collenchyma. From HABERLANDT. 

 pears that certain stems 



when exposed to tension for some days or weeks become somewhat 

 stronger; furthermore, the bast fibers and hadrome elements become 

 somewhat more numerous and have slightly thicker walls than in 

 controls not so exposed. In many cases tension appears to cause no 

 appreciable change. 



In pendent fruits (as the apple) the tension on the fruit stalk increases constantly 

 as the fruit gains in weight, also the strength of the stalk increases from week to week, 

 although it has been shown that the increasing pull of the fruit is not a determin- 

 ing factor of importance. The fruitstalks of Cucurbila, however, have mor 



