440 



BOTANICAL GAZETTE 



[may 



is a marked retardation in the development of the pairs of rays the ap- 

 pearance of the deeply depressed segments is coincident with that of 

 the rays (fig. 4). Particularly significant are those stems in which 

 one ray of a pair fails to develop. Under these circumstances, the nar- 

 row segment of xylem tends to be unsymmetrically depressed (fig. 4). 

 Furthermore, the fact that depressed segments may occur between 

 pairs of rays, which are opposite the projecting lobes of the pith (fig. 3), 

 and between approximated " secondary " rays, suggests that the stellate 

 outhne of the early cambium is not an indispensible factor in the pro- 

 duction of the sunken wedges of xylem in oak stems. 



Fig. 3 



Fig. 



Miss Langdon offers a physiological explanation for the stellate form 

 of the stele in young twigs (p. 3 2 1 ) : 



Since the principal function of the xylem is the conduction of water from 

 the soil to the outer parts of the plant, it is obvious that the maximum upward 

 movement of solutions in the stem would be through the tracheidal tissues and 

 vessels in direct line with the leaf traces. This would cause an acceleration in 

 growth and the consequent outward projection of those five regions of the woody 

 cylinder associated with leaf traces, while the neighboring conducting tissues, 

 namely, the so-called depressions from which the main conducting streams 

 had been diverted to the petioles of the leaves, would fail to maintain their 

 normal rate of growth. 



It is to be emphasized, in this connection, that the projecting wedges 

 of the first annual rings of Castanea, Populus, and Quercus, when devoid 

 of wide rays, are not due to an acceleration of growth. This can readily 

 be determined by measuring the depth of the convex and concave arcs of 



