45 



part where it is certain no dead cells existed when the gypsum was 

 laid around the stem. But in such cases as these the peripheral zone 

 of living pith was greater within than outside the cast, and in those 

 species where an entirely living pith was not preserved within the 

 casts there was more living pith than in normal parts of the stems. 

 Thus is it shown that the pith will live a short or long period 

 heyond the usual time according as the casts are applied shortly 

 or long before the cells would normally die. 



It cannot be said that the pith in any of the plants under ex- 

 periment would normally live much longer if not torn by the cortex. 

 In all experiments where casts have been applied shortly before the 

 time for tearing, the life of the pith has been prolonged but a few 

 days. 



4. Effect of gypsum casts on the growth of supporting 

 and conducting tissue. 



The most striking result obtained in these experiments is the 

 lack of formation of mechanical tissue within the casts. It is self- 

 evident that a stem whose growth is so confined that multiplication 

 of cells cannot take place will not be able to form so much mechan- 

 ical tissue as a normally growing stem. But a plant can be stimu- 

 lated to increase more than normally the thickness of its cell-walls 

 as has been demonstrated by the researches of Elfving 1 and Hegler 2 . 

 The former found that plant stems when forcibly bent showed the 

 collenchyme on the convex side thicker-walled and on the concave 

 side thinner- walled than normal. The latter by growing plants under 

 a stretching force was able to call forth an increase of the various 

 kinds of mechanical tissue accompanied by a greater thickness of 

 cell-wall. It might be supposed that when within the plaster casts 

 new cells could not be formed, the preexisting immature cells would 

 become through regulation thicker-walled than usual. But the reverse 

 is true. Cells formed before the casts are applied which normally 

 become thick- walled remain thin-walled within the gypsum. 



In none of the stems surrounded by gypsum has growth either 

 by division or extension of cells been immediately brought to a 

 standstill. As mentioned earlier, the room for this growth is first 

 furnished by the intercellular spaces. After these are filled there 



1 Elfving, Zur Kenntniss der Krummungserscheinungen der Pflanzen. Sartryck 

 ur Ofersigt af Finska. Vet. Soc. Forhandlingar. Helsingfors 1888. 



2 Hegler [in the review by Pfeffer previously mentioned]. 



