august] BOTANICAL GAZETTE 112 



safranin and gentian violet, or safranin and "licht grim," the latter 

 combination proving the more satisfactory. 



The greater part of the study of the girdling habit in the seed- 

 lings was made from cleared material. Entire sections comprising 

 stem cylinder and cortex, in blocks 0.5X1 inch, were cleared so 

 perfectly that it was possible, with the aid of a strong artificial 

 light, to look through such a section and see the vascular strands 

 clearly outlined in the cortex. 



In the case of the two- and three-year-old seedlings, the method 

 followed consisted in severing the long taproot from the stem just 

 below the region of the cotyledonary plate and cutting off the long 

 terminal leaves, leaving only the leaf bases and a small part of the 

 petiole. The scale leaves were then carefully trimmed from two 

 sides of the stem, and one clean longitudinal cut made through the 

 entire stem from apex to base. After the transference of these 

 half-sections to 50 per cent alcohol (each seedling being kept in a 

 separate receptacle), the process was substantially the same as 

 that for the paraffin method, that is, up to the pure xylol stage. 

 At this point the material was subjected to vacuum treatment in 

 order to free the tissues, as far as possible, of any .air or gases they 

 might contain. As a final clearing agent a mixture of xylol and 

 carbon disulphide was used; the CS 2 , having a higher refractive 

 index, rendered the material more transparent than pure xylol. 



Adult stem 



With the single exception of the Australian Macrozamia Hopei, 

 Dioon spinulosum Dyer is the tallest of all cycads, ranging 10-30 

 ft. in height, with occasional specimens reaching 40 and even 50 ft. 

 The particular specimen from which this study was made was 

 about 18 ft. tall and possibly 100 years old. The width of the 

 woody zone from pith to cortex averages 0.5-0.75 inch in the 

 upper part of the trunk and 3.5-4 inches in the lower trunk. 



Structure of xylem. — The adult stele of D. spinulosum 

 is endarch, and its compact woody cylinder consists chiefly of 

 longitudinal tracheidal elements and radial parenchyma. From 

 the pith to the cortical par* of the stem the length of the tracheids 

 averages as follows: scalariform metaxylem tracheids 4-4.2 mm., 



