RICHARDS. DEVELOPMENT OF iECIDIA. 259 



The septatiou of the basidia which started in the older ones is con- 

 tinued by the younger ones near the periphery, and the next definite 

 change that is to be noted is the beginning of the peridium. The 

 formation of the peridium commences by the differentiation of the 

 terminal cells cut off from the older basidia. Their walls become 

 definitely thicker and their contents very vacuolate, presenting quite a 

 distinct difference from the spore mother cells below (Fig. 3). As the 

 other young spore chains push up, their terminal cells also become 

 similarly altered and the formation of the peridium progresses rapidly 

 from the centre outwards. The peridial cells so formed increase 

 more rapidly in size than the spores below, and owing to this and the 

 interpolation of new basidia, which grow up from the base, their 

 connection with the chains of cells from which they originated is 

 often lost. Sometimes, however, this connection is retained quite 

 clearly even when the aecidium is well along in its development, due 

 perhaps to the fact that in such cases the young basidia have grown 

 up more nearly simultaneously than usual and that there has been 

 no intercalary formation of basidia. At times the enlargement of the 

 peridial cells is more rapid than need be to cover the hymenium 

 as it enlarges, in which cases they may be pushed out of place so 

 as to give the appearance of a double layer of cells. 



This metamorphosis of the terminal cells continues to the very mar- 

 gin of the hymenium, and there the same modification affects the whole 

 peripheral row of spore chains, so that the aecidium is now completely 

 incased in a covering of modified sporei which may be called the perid- 

 ium. This last point is shown better in other aecidiathan the one now 

 under discussion, and will be referred to later. Nothing was seen 

 that would lead one to suppose that the peridium originated around 

 the periphery and grows up until it meets in the centre, nor did it 

 appear that it is derived in any way from the pseudo-parenchyma 

 layer, for the line of demarcation between this and the peridium is 

 always apparent (Fig. 3). 



Concerning the formation of the spores there is not much that is 

 new to be said. They are cut off basipetally from the basidia or 

 foot cells, and the cells thus formed are being constantly pushed up 

 by the formation of new ones below. The upper row forms, as has 

 already been described, the peridium ; from the others the spores are 

 produced. In these cells may be seen a double nucleus, or rather two 

 nuclei almost always in close proximity to each other (Fig. 5, a). These 

 have already been described by Rosen,* and also by Dangeard, but as 



* Cohn's Beitrage, VI. 



