32 



than above or below the cast. The measurements on which this 

 estimate is made were taken with an ocular micrometer on an imagi- 

 nary line passing through the middle of the zone of xylem. Only 

 complete cross-sections were used for this purpose. The irregular 

 course of the zone in the part of the stem within the cast renders 

 a computation by comparison of radii impossible. The only other 

 factor needed is the average number of the xylem- cells in a radial 

 row, since the size of the cells is about the same within and without 

 the cast. These 2 cases thus measured are not exceptional, as the 

 eye can easily detect the difference in amount of xylem in the normal 

 and abnormal parts. The differences obtained are so large as to ex- 

 clude the possibility of changing the general result through a more 

 accurate computation. 



The relations between the tissues within and without the casts 

 can be conveniently expressed for the pith and the mechanical xylem 

 in the following table: 



Internodes added 



after the application 



of cast 



Living pith-cells in 



a radius 



I No thick-walled xylem-cells 

 in a radius 



Bolow 



Within 



Above 



Below 



Within 



Above 





the cast 



the cast 



the cast 



the cast 



the cast 



the cast 



1. 3 internodes added 



14 



16 



13 



10 



10 



8 



2. 4 internodes added 



15 



16 



14 



12 



14 



10 



3. 5 internodes added 



18 



27 



18 



15 



30 



15 



4. 8 internodes added 



12 



18 



12 



20 



29 



20 



5. 10 internodes added 



25 



30 



25 



25 



45 



25 



JRicinus communis L. 



In this plant throughout the stem except in the hypocotyl the 

 cavity is formed before elongation is complete. In the hypocotyl 

 however the cavity appears first in the highest part after elongation 

 ceases and progresses very slowly downward. One might expect there- 

 fore in these 2 parts of the plant to find the pith-cells concerned in 

 cavity-formation behaving differently. But the early stages of this 

 process are really very similar in the place where the ring of bundles 

 is rapidly moving from the centre and where such movement has 

 ended. In both places the pith before the cavity appears has inter- 

 cellular spaces of moderate size filled with air. In the parts of the 



