PHYSIOLOGICAL INTEGRATION IN PLANTS. 279 



panied by functional indefiniteness. Though, considered 

 collectively, these Thallogens form " three very natural 

 groups, according as they inhabit the water, the earth, or 

 the air ;" yet if, instead of their higher members we look at 

 their lower members, we find these distinctions of habitat very 

 undecided. Alga, which are mostly aquatic, include many 

 small forms that frequent the damp places preferred by 

 Lichens and Fungi. Among Lichens, as among Fungi, there 

 are kinds that lead submerged lives like the Alga. While 

 terrestrial Lichens and Fungi compete for the same places, as 

 well as simulate one another's modes of growth. Besides 

 this indistinctness of the classes, there is great variability in 

 the shapes and modes of life of their species — a variability 

 so great that what were at first taken to be different species, 

 or different genera, or even different orders, have proved to 

 be merely varieties of one species. So inconstant in struc- 

 ture are the Algce that Schleiden quotes with approval the 

 opinion of Kutzing, that " there are no species but mereh/ 

 forms of Algce." In all which groups of facts we see that 

 these lowest types of plants, little differentiated, are also but 

 little integrated. 



Acrogens present a parallel relation between the small 

 specialization of functions which constitutes physiological 

 differentiation, and the small combination of functions which 

 constitutes physiological integration. "Mosses," says Mr. 

 Berkeley, "are no less variable than other cryptogams, 

 and are therefore frequently very difficult to distinguish. 

 JNot only will the same species exhibit great diversity 

 in the size, mode of branching, form and nervation of the 

 leaves, but the characters of even the peristome itself are 

 not constant." And concerning the classification of the 

 remaining group, Filicaks, he says : — " Not only is there 

 great difficulty in arranging ferns satisfactorily, but it is 

 even more difficult to determine the limits of species/' 



After this vagueness of separation as well as inconstancy 

 of structure and habit among the lower plants, the stability 



