PHYSIOLOGICAL INTEGRATION IN PLANTS. 295 



the succulent leaves of B. calycinum, borne on foot-stalks 

 so brittle that they are easily snapped by the wind, send 

 forth from their edges when they fall to the ground, buds 

 which root themselves and grow into independent plants. The 

 correlation here obviously furthering the preservation of the 

 race, is more definitely established in another species of the 

 genus — B. proliferum. This plant, shooting up to a consider- 

 able height, and having a stem containing but little woody 

 fibre, habitually breaks near the bottom while still in flower; 

 and is thus generally prevented from ripening its seeds. The 

 multiplication is, however, secured in another way. Before 

 the stem is broken young plants have budded out from the 

 pedicels of the flowers, and have grown to considerable lengths ; 

 and on the fall of the parent they forthwith commence their 

 separate lives. Here natural selection has established a 

 remarkable kind of co-ordination between a special habit of. 

 growth and decay, and a special habit of proliferation. 



§ 285. The advance of physiological integration among 

 plants as we ascend to the higher types, is implied by their 

 greater constancy of structure, as well as by the stricter limi- 

 tations of their habitats and modes of life. " Complexity of 

 structure is generally accompanied with a greater tendency 

 to permanence in form," says Dr. [now Sir J.] Hooker; or, 

 conversely, " the least complex are also the most variable." 

 This is the second aspect under which we have to contem- 

 plate the facts. 



The differences between the simpler Algce and Fungi are 

 so feebly marked that botanists have had great difficulty in 

 framing definitions of these classes. This structural indefi- 

 niteness is accompanied by functional indefiniteness. Algce, 

 which are mostly aquatic, include many small forms that 

 frequent the damp places preferred by Fungi. Among Fungi, 

 there are kinds which lead submerged lives like the Algce. 

 Besides this indistinctness of the classes, there is great varia- 

 bility in the shapes and modes of life of their species — a vari- 



