lis PROCEEDINGS OP THE AMERICAN ACADEMY 



of small spherical cells provided here and there with definite hetero- 

 cysts. Each of these chains is embedded in a definite, more or less 

 spherical mass of jelly, and by any one familiar with the types of the 

 lower algse they are recognized as unaltered Nostoc colonies. 



With the exception, then, of the fact that the hypbal nature of the 

 thallus still preponderates slightly over that induced by the gelatinous 

 sheaths of the Nostoc colonies, this lichen presents us with a structure 

 identical with that seen in Leptogium, 



It is a matter of very little difficulty to find the earliest stages of 

 the reproductive organs, both spermogonia and apothecia. They are 

 mostly limited to separate thalli, and are densely crowded, the apo- 

 thecia i^articularly, occupying the centre of the lobe in large numbers, 

 the edges being comparatively free from them. If then a series of 

 sections be made progressively from the edge towards the centre, the 

 different developmental stages can be clearly traced. It occasionally 

 happens that near the edge of a thallus lobe, otherwise provided with 

 apothecia only, there occur a very few spermogonia, and in such cases 

 a comparison of the early stages of the two organs lying side by side in 

 the same section is very instructive. Nothing further need be added 

 here to the course of spermogonial development already described. 



The apothecial primordia differ considerably from the youngest 

 spermogonia. In size they are rather larger, in shape they are 

 much more spherical, and in position they are more deeply sunk in 

 the thallus, so that very few algal colonies occur below them. The 

 structure and origin of the small, spherical masses of interwoven hy- 

 pha3 forming the primordia are easily made out, and although it seemed 

 highly probable, from the close relationship evidently existing between 

 this lichen and the Collemacete, that here would be found some form 

 of sexual reproduction analogous to that described by Stahl in Col- 

 lema, I was unable to find any trace of such a condition. Before the 

 simple character of the young apothecium has begun to give place to 

 the more complex condition seen later, the thalline elements above it 

 begin to disorganize. What this result is due to it is impossible to 

 state. The growth of the primordium undoubtedly exerts a tension 

 on the overlying tissue, but the disintegration seems rather to be 

 caused primarily by the death of the protoplasmic contents of the cells, 

 and the absorption of the membranes, in a manner similar to that 

 which produces the central cavity in the young spermogonium. But 

 from whatever cause, by the time the first paraphyses have arisen 

 from the upper part of the primordium, the overlying tissue has dis- 

 appeared, and the young hymenium, though still deeply sunken in the 



