BOT.— Vol. I.] PEIRCE— NATURE OF LICHENS. 229 



closely applied to a gonidial cell which has begun to divide. 

 Figure 20 is from a similar section of Sfcerofhoriis and 

 demonstrates the intimacy of contact between the hyphae 

 and a gonidial cell also dividing. I did not succeed in 

 finding haustoria in these two lichens, but from the close- 

 ness of the contact which, as these two figures show, exists 

 between hyphse and gonidia, and from the foregoing dis- 

 cussion of the physiology of this relation, one is certainly 

 justified in concluding that the fungus and the alga stand to 

 each other, in these two lichens as in Ramalina, in the 

 relation of parasite and host. The algse in these three 

 lichens are the same, Cystococcus humicola, Nag.^ It would 

 be interesting to compare with these forms lichens which 

 have other species of algee as gonidia, but for various 

 reasons I prefer to leave this to others or to later work of 

 my own. 



V. The Significance of the Water-content of 

 Ramalina reticulata. 



The amount of water held, and the force with which it is 

 held, even in very dry air, is significant. The lichen cannot 

 die from drought, and the alga is defended against extreme 

 dryness by the gelatinous enveloping fungus, which holds 

 water enough at least to keep the alga alive, and probably 

 enough to enable it to elaborate some food each day. It is 

 hard to conceive that the alga, supplied with any moisture 

 at all, should be entirely inactive when daily illuminated by 

 the sunlight; and owing to the affinity of the gelatinous 

 outer hyphse for water, the dews and the frequent fogs at 

 night must furnish as frequent additions to the water-content 

 of the lichen. It seems extremely probable, therefore, that 

 the long dry season of California, checking the growth and 

 many other activities of most plants hereabouts, is not suf- 

 ficiently dry to stop the photosynthetic and other activities 



1 Professor William A. Setchell, of the University of California, kindly informs me that 

 this alga is now commonly called Chlorococcum humkola (Rab.) Nag. 



