CALLOSE IN ROOT-HAIRS 



117 



thickening substance could be clearly seen in optical sections, and 

 the absence of definite pits, such as usually appear in other thick- 

 ened cell walls, could be determined (fig. 1). 



That no definite mle could be established concerning the dis- 

 tribution of the thickened hairs on the roots was demonstrated 

 by the examination of a large number of preparations. They 

 were found as frequently upon the younger as upon the older parts 

 of the roots (figs. 1, 6, 8 and 9) and occurred singly or in groups of 

 half a dozen or more. Some of those on the older portions of the 

 roots gave evidence of the formation of callose with consequent 

 cessation of elongation during the early period of their growth 

 (fig. 1), while others (figs. 2, 3, 4, 5, 6 and 9) showed clearly that 

 the thickening substance had been laid down after various later 

 stages of growth had been reached. In some cases such hairs 

 were found distributed almost uniformly on all the roots of the 

 plant, in other cases they were found on some roots while others of 

 the same plant did not show them, and finally, on plants growing 

 close to one another, apparently of the same age and under the 

 same conditions, the roots of one would show the thickened hairs 

 in large numbers while those of its neighbor had very few or even 

 none at all. 



With regard to the nature of the thickening, there appeared to 

 be no uniformity except that the substance laid down on the 

 inside of the cell wall was, with a single exception, the same, 

 namely, callose. In one instance the thickening substance failed 

 to give the reactions for callose (fig. 12). In this case the pad 

 resembled in appearance the sort of thickening figured by Kiister, 

 and it also gave definite cellulose reactions. The callose thicken- 

 ing was found uniformily throughout the hair and on the outer 

 and radial walls of its basal part in some, in a varying degree, 

 from a very thin layer to a complete closure of the lumen (figs. 

 1, 3, 6, 8 and 9).' In others it occurred in the distal (figs. 2, 3 and 

 5) or in the proximal portion only; in both distal and proximal 

 ends with Uttle or none in the middle portion; as one or more par- 

 tial or complete plugs in the lumen (figs. 2, 4 and 6) or as very 

 irregular deposits throughout the whole haii* except on the inner 

 tangential wall of its base (fig. 9) . In a few cases pads or lumps of 



THE PLANT WORLD, VOL. 16, MO. -1, APRIL, 1913 



