41 



Within the casts the parenchyma cells of the fundamental tissue 

 not only remain smaller and thinner-walled but their growing period 

 and life-period are longer than normal. Zea mais and Cucurbita pepo 

 which develop thik-walled sclerenchyma cells from their fundamental 

 tissue show within the casts such cells slowly increasing the thickness 

 of their membranes after corresponding normal cells in neighboring 

 parts of the stems have ceased to grow. All plants in which the 

 outer part of the pith parenchyma normally becomes thick-walled 

 have shown, within the casts, this tissue always in a much thinner- 

 walled state, but the membranes slowly increasing in thickness after 

 the permanent condition in corresponding cells above and below the 

 casts had been reached. Such plants are Vicia faba, Cucurbita pepo, 

 Melianthus major and Dahlia variabilis. 



All the plants that have been under experiment, except the one 

 species Ricinus, have given positive evidence that the application of 

 the cast before primary extension has ended prolongs the life-period 

 of cells. Ricinus has shown within then cast dead crushed cells in 

 the cortex as early as dead cortical cells were found outside the 

 cast. But the normal cortex was then very spongy, the cells hanging 

 together with but slight connectione and certainly incapable of resist- 

 ing such a pressure as was being exerted on the cortex within the 

 gypsum. Is is fair then to assume the possibility of the pressure 

 from the region of the cambium crushing these cortical cells earlier 

 than they would have died without such a crushing force. Thus 

 Ricinus does not necessarily form an exception to the behaviour of 

 the other plants examined in that they preserve longer the life of 

 those cells which are not allowed to take their full size. 



Except in the cortex of Ricinus communis all organs which nor- 

 mally become hollow or in which there is destruction of tissue have 

 by the application of casts had such tissue longer preserved than in 

 normal parts of the organ above and below the cast. This has 

 proved true for the middle parenchyma of leaves of Allium cepa ; 

 for the central tissue of the petiole Ricinus communis; for the cells 

 whose collapse forms the intercarinal canals of Equisetum limosum ; 

 for the cells which break down to form the air-canal in Zea mais ; 

 for the "pith" of the rhizome of Triticum repens ; for the cortex of 

 Vicia faba, Laminum garganicum, Urtica dioica, and Melianthus 

 major ; for the pith of Caltha palustris. Althaea tauriensis, Cucurbita 

 pepo , Larnium garganicum , Urtica dioica , Vicia faba , Archangelica 

 sativa . Myrrhis odorata , Dahlia variabilis , Ricinus communis and 

 Melianthus major. 



The effect of applying casts in prolonging the life of the pith 



