406 



UREDINEAE. 



the same autumn; they are small, one-pointed, and pale from 

 an almost complete lack of chlorophyll. In these respects they 

 are quite distinct from the larger double-pointed normal needles 

 with their dark-green colour and a period of growth extending 

 over several years. All the needles on a broom are as a 

 rule stunted in the manner described, yet single branches 

 may be found with needles quite normal ; such contain no 

 mycelium, or, if so, it has found its way in too late to have 

 any effect on their growth. 



Fig. 2i^.— Witches' Broom oj Silver Fir (summer condition). The murkedly 

 negative geotropic broom has its origin in a distinct basal swelling, (v. Tubeuf 

 phot.) 



The various tissues of the witches' brooms also underao 

 considerable modification as compared with normal twigs. A 

 thicker and softer bark is present, due to the parenchymatous 

 cells of both outer rind and bast having enlarged in size and 

 increased in number ; the cork layers are also abnormally in- 

 creased. The same changes may be observed in the rind of the 

 swellings, and to this their increased size must be chiefly ascribed. 

 The wood both in twigs and swellings is much increased ;. 

 the year-rings however are very variable, sometimes they are 

 broader than the normal, again they may be diminished or 

 even altogether wanting ; where however the wood decreases, 

 there the bast increases in proportion. This lack of uniformity 



