200 



PLAXT LIFE. 



when grasses are lilo-\vn down or tramijled their stems erect 

 themselves by the geotropism of this basal growing zone 

 (fig. 196). 



Fic;. igiS, — Part of a wheat-stalk, slicnving strong geotropic cur^-ature. I'he slioot was 

 placed horizontal, and tlie growth of the basal part of the internode with the leaf-sheath 

 connected with it was stimulated on the under side, the upper remaining short. No 

 ciir\-ature occurs in the older part of the internode. About two thirds natural size. 

 —After Pfeffer. 



Fig. 1Q7. — Root-cage. (Jn the lower edge of a sheet of zinc a little larger than the panes 

 of glass selected is formed a water-tight trough of the same material. Two panes of 

 glass of suitable size are clamped together, with a piece of wood i cm. thick on three 

 edges to keep them separate, Seeds are sown in fine s<iil ex'cnly packed between the 

 panes; these are set with the lower edge in the watei'-trough and a sheet of zinc is 

 used to keep out light. The cage should be slightly inclined, as shown, so as to keep 

 roots against the glass. — From a drawing by J. C. Arthur. 



