25 



usually growing as well as normal ones by their side; nearly always 

 the effect of the cast on the cellular structure extends less than 

 5 mm below and 10 mm above its limits. Still it can be said that 

 there is a tendency in such plants to the production of a slightly 

 weaker individual. Since the most of the growth in thickness takes 

 place in this plant before elongation is complete, casts must be ap- 

 plied before elongation is complete, and as a result the swelling out 

 of the stem above the cast is gradual, not sudden. The experiments 

 fall into 2 classes: those in which the casts were applied before 

 the cavity in the stem was present, and those applied after the ap- 

 pearance of the cavity. 



With Casts applied before the Presence of a Cavity. 



For this series of experiments 7 plants were put into casts, the 

 plants exposing at the time but 3 internodes, and the cast enclosing 

 the 3rd internode and all or a part of the 2nd. Plants similar to 

 those encased in gypsum were preserved in alcohol for future com- 

 parison. 



1. 16 days after casting. A study of control-plants of like age 

 and development with the one from the cast shows that the normal 

 changes in these 16 days have consisted in the enlargement of 

 cells generally, in the formation of greater intercellular spaces, in 

 the addition of many vessels to each bundle, and in the formation 

 of a central cavity in all internodes above the epicotyl. The cam- 

 bium-ring has at this period not been completed and no mechanical 

 cells have appeared in the bundles. 



These normal changes are present in the plant from the cast, 

 above and below the limits of the gypsum. Within the cast the 

 cells have become but slightly larger , the intercellular spaces 

 smaller, and the vessels more numerous than 16 days earlier. The 

 vessels within the cast are smaller, crumpled and number about 8 in 

 an average bundle. Above the cast they are larger, of normal size 

 and number about 20 to a bundle. The cast included the 2nd epico- 

 tyledonary internode and hence there is no cavity below. Within 

 is no cavity ; above is a cavity about y 4 the diameter of the stem. 

 Within, the cells of the cortex, especially in the outer rows are 

 elongated radially, and as everywhere else in the stem the cambium 

 between the bundles has not formed. 



2. 33 days after application of cast. The normal progress of 

 growth up to this time showed these features, in the 2nd and 3rd 

 internodes: The cambium-ring had been completed , and had given 



