264 TROCEEDINGS OF THE AMERICAN ACADEMY. 



Aecidiuji on Sambucus. 



In this jEcicIiiim there is a considerable amount of distortion of the 

 tissues of the host, but at present only the Eecidiuiu itself will be 

 considered. It did not afford a very good chance for studying the 

 younger stages, but, as nearly as could be made out the development 

 corresponds more nearly to that of the tecidium on Ranunculus 

 than to any other. 



The peridium begins in the usual way from the metamorphosis of the 

 terminal cells in the older spore chains, and spreads over the top of 

 the hymenium, as already described. The fact that the side walls ot 

 the peridium correspond to tlie outer row of spore chains is excellently 

 shown in this species. The peridial cells, wliich become smaller and 

 thinner walled as they approach the base of the hymenium, are seen 

 finally to merge into a foot cell which exactly resembles the other 

 basidia. In the youngest peridial cells even the interstitial cells were 

 observed, although De Bary in his text-book says that they are not 

 present in the peridium (Fig. 13). Very soon, of course, owing to 

 the enlarging of the peridial cells and to the thickening of their walls, 

 the interstitial cells disappear sooner than they do between the spores. 



Passing to the vEcidium known as Peridermiwni elatinum^ we find an 

 interesting variation in the formation of the primordium. What may 

 be properly called the primordium in this case is but poorly developed, 

 consisting of sim{)ly a mass of loosely arranged pseudo-parenchyma 

 of rather large cells, and with none, or practically none, of the hyphal 

 weft about them. It is not improbable that this form might prove 

 a very excellent one for studying the young stages of the hymenium, 

 and it is greatly to be regretted that the material at hand was too 

 advanced for that purpose. The formation of the peridium, however, 

 showed admirably. In Figure 14a young a^cidium is shown, in which 

 two peridial cells have just begun their differentiation from the spore 

 chains. The mature aecidium, aside from the necessary reduction of 

 the mass of collapsed pseudo-parenchyma outside of the peridium, 

 showed no great peculiarities. 



The last ascidium to be considered is Roestelia. It was hoped 

 that, owing to the long period of formation that the Roestelia! un- 

 dergo before they emerge through the tissues of their hosts, it n-ight 

 be coupled with some longer preparatory growth of the a^cidium 

 itself. Such, however, did not prove to be the case, for in all of the 

 species examined, although the origin of the Roestelia was very deep 

 in the tissues of the host, and consequently it was a long time in 



