47 



found thinner-walled than in normal parts of the stem, while within 

 the cast but near its limit the corresponding cells though no larger 

 than farther within the cast will he thicker-walled than such cells 

 in normal parts of the stem. It should be added also that the ab- 

 normal increase of mechanical tissue is greater at the upper limit 

 of the cast than at the lower. The difference is probably to be 

 accounted for from the fact that primary growth when the cast was 

 applied was more nearly ended as the lower limit than at the upper, 

 and hence at the upper as indicated by the evidence of gliding 

 growth there has been more tension of tissue. Tension as a stimulus 

 to formation of mechanical tissue has been previously discussed in 

 this paper. 



All kinds of mechanical tissue found in the plants that have 

 been under experiment have remained more or less undeveloped 

 within the casts: the collenchyma of Archangelica and Myrrhis ; 

 sclerenchyma of Triticum , Zea , Caltlia and Cucurbita ; hard bast, 

 xylem elements, and pith of all plants studied. In all the plants 

 with the possible exception of Urtica , the cells which normally 

 become hard bast are the last to show any thickening of membrame 

 within the cast. 



As was found true for the parenchyma cells, so also the mechanical 

 cells retain longer within than outside the cast the power of growth. 

 Yet eventually the cells that normally become mechanical must 

 within the cast pass into their mature stage without reaching their 

 normal development. When a segment of a stem during primary 

 growth is confined in gypsum for several weeks, the remainder of 

 the stem growing meanwhile, the confined segment will not elongate 

 on being released from the cast, as has been proved experimentally 

 in several cases. The cells have matured without reaching their 

 normal length. 



With the possible exception of Triticum , Zea and Melianthus 

 all plants that produced enough mechanical tissue within the cast 

 to allow a comparison to be made showed in cross-section the 

 elements of such tissue smaller than normal elements. Thus in 

 horizontal diameter also, the mature condition may be reached with 

 an extension less than normal. This conclusion was reached by de 

 Vries and Krabbe for tracheids and vessels of woody plants ; it would 

 seem to be general for all cells. 



It is not improbable that cells normally mechanical will mature 

 at a thinner-walled condition than usual , though the study in this 

 direction has been conclusive in but 2 cases. It is difficult to de- 

 termine from observation of sections when cells within the con- 



