56 DEVELOPMENT AND HEREDITY. 



microscopical examination, that the mesophyllum, the 

 upper side of the twig, is differently constructed 

 from that of the lower side. The green cells on the 

 upper side are rich in chlorophyll bodies, and have 

 a pallisade-like form : the cells on the under side 

 are poor in chlorophyll, and have a nearly iso-diamet- 

 rical form. The dorsi-ventral structure of the Thuja 

 twig expresses itself in other ways, which, however, 

 we will not discuss here. Let the Thuja twigs be 

 taken in early spring, before the growth of the buds 

 has started, and without separating the twigs from the 

 plant, let them be tied in an inverted horizontal posi- 

 tion, so that the natural under side is turned toward 

 the zenith, — it will then be found, as was first 

 shown by Frank, that the year's additional growth 

 certainly assumes a dorsi-ventral nature, but is essen- 

 tially different in its anatomical structure from those 

 parts of the twig which were of previous growth. 

 The upper side of the newly grown parts of the 

 twig, although it is the continuation of the under 

 side of the older part, has an anatomical structure 

 like the upper side of the older part. Thus, the 

 dorsi-ventral structure of the Thuja twig, which 

 appears in the development of palisade-parenchym 

 on the upper side, and soft parenchym on the under 

 side, is the result of the working of an external 

 force ; and in this case, according to all that we 

 know, we must regard the light as the cause. 



