UREDINEAE AND HYMENOMYCETES 529 



the teleutospore-sorus with a good chance of escaping into the air 

 so that they may be carried off by the wind, it is necessary that, 

 immediately before their discharge, they shall be directed away from 

 the sorus. 'The proper orientation of the sterigmata in space is 

 accomplished in the Uredineae in the following manner. When 

 the teleutospores germinate, the germ-tubes at first grow in a direc- 

 tion more or less parallel to the axis of the teleutospore and towards 

 its exterior, i.e. more or less perpendicularly outwards from the 

 surface of the teleutospore-sorus ; but, on emerging from the sorus, 

 they soon begin to grow at their apices more rapidly on one side 

 than on the other. In consequence of this differential lateral 

 growth, which appears to be in no way dependent on external 

 stimuli, the end of each basidium becomes curved. This curvature 

 is usually arcuate in form and the extreme end of the basidium often 

 becomes bent, with reference to the basidium-base, through an angle 

 of 90 or more degrees. The convex side of the curved part of the 

 basidium is directed away from the sorus and the concave side 

 toward the sorus. A basidium, as soon as it has assumed its sickle- 

 shaped form, ceases to grow in length and becomes divided by 

 septa into four cells. Four sterigmata, one from each cell, are then 

 developed on the convex side of the basidium and each sterigma 

 soon gives rise to a terminal spore. Thus the sterigmata and the 

 spores become more or less directed away from the sorus-surface 

 (c/. Figs. 215, I and 216 ; pp. 535 and 537). In consequence of this 

 arrangement, the spores when shot away from their sterigmata, 

 stand an excellent chance of leaving the substratum on which the 

 teleutospores have been developed, and of being caught and carried 

 off by the wind. The explanation of the sickle-shaped form of the 

 basidium in the Uredineae is now obvious : it is a form ivhich secures 

 that the basidiospores shall he shot away from the teleutospore-sorus in 

 a direction favourable to dispersion. Were the basidia not curved, 

 or were the sterigmata produced on the concave side of the basidium, 

 it is obvious that basidiospore-discharge could not be anything 

 like so successful as it actually is. In order to make clear the 

 position of the basidiospores on the curved basidia in relation to 

 the sorus as a whole, I have undertaken a special investigation 



upon Puccinia Malvacearum. The details of the investigation will 

 VOL. iir. 2 M 



