82 RELATION OF PLANTS TO GRAVITATION 



solution of osmic, acetic and chromic acid, dehydrate, wash, in- 

 filtrate and imbed in paraffine, a process that takes a few days. 

 Cut the root into a series of longitudinal sections, using an auto- 

 matic microtome and fasten to glass slips. 1 Dissolve out the 

 paraffine and bring the material by proper stages through mix- 

 tures of alcohol and xylol into 40 per-cent. solutions of alcohol. 

 Stain in a solution of Bismark brown in 4<D-per-cent. alcohol and 

 return by proper stages to pure xylol and mount in Canada bal- 

 sam. Now measure the distance from the tip to the region of 

 curvature. Compare the cells of the tissues in correspondent 

 positions on the convex and concave sides of the organ. Meas- 

 ure carefully and note staining reaction on the walls and ecto- 

 plasm. What differences in quality, size and contour are to be 

 seen ? Make similar sections through a straight root and com- 

 pare size and form of cells. 2 



124. Diageotropism of Secondary Roots. Germinate some 

 seeds of the pea or bean in moist sawdust until a number of sec- 

 ondary roots have been formed. Now remove the seedlings and 

 place in a moist chamber with the main root horizontal, or put 

 in the same position in the sawdust. Either preparation should 

 be kept in the dark room. Two days later note the position of 

 the secondary roots. These organs will be found to occupy a 

 position departing more or less from the horizontal and constitut- 

 ing diageotropism. 



125. Sensory Zones of Shoots. The entire growing region of the 

 shoot of many seed plants may be capable of appreciating geotropic 

 stimuli, and it is not possible to make such sharp delimitation as in 

 the case of the sensory zones of roots. The sensory function may 

 be more highly developed in some tissues than in others, generally 

 reaching its greatest delicacy in the cortical cells, though the 

 pith is capable of both perception and reaction in some species, 



1 Zimmerman. Botanical Microtechnique. 1893. 

 2 MacDougal. Curvature of roots. Bot. Gazette, 23 : 340. 1897. 

 See also Pollock, J. B. The mechanism of root curvature. Bot. Gazette, 29: i. 

 1900. 



