CH. VII] GRASS-HAULMS. 165 



(193) Subsequent change in the form of curvature 1 . 



If specimens prepared as in exp. 192 are left 

 undisturbed for some hours longer, the free end of the 

 shoot will be carried far beyond the vertical. A record of 

 this movement and the subsequent return to the vertical 

 may be made, by using a box with one vertical glass 

 wall ; if the shoot is fixed (in a sand-embankment) so 

 that it is close to the glass, its changes of form can be 

 traced with a paint-brush filled with black varnish on the 

 glass. Sachs' large diagrams published with Vol. III. of his 

 Arbeiten should be consulted. 



(194) Grass-haulms 2 . 



Cut a grass- haulm which is vertical and in which the 

 pulvinus has not curved. Mark the pulvinus on two 

 opposite faces by means of dots some 2 or 3 mm. apart. 

 Having measured the distance between the marks, push 

 the haulm horizontally into a sand-embankment in a tin 

 box so that one set of marks are above, the other below. 

 After 24 hours or more the haulm will have bent at the 

 pulvinus, when the marks must be again measured. The 

 lower surface will have increased greatly in length while 

 the upper surface has become shorter. 



Note the pale colour of the lower half of the pulvinus ; 

 and if the pulvinus is a hairy one, note the divergence of 

 the hairs below and their convergence above. 



1 Sachs' Collected Papers, n. p. 967 (from Flora, 1874). 



2 Sachs' Collected Papers, n. p. 958 (from the Arbeiten, i.). 



