278 PHYSIOLOGICAL UE^-ELOPWEKl 



80 brittle tliat they are easily snapped by the vviuil, send 

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

 that 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. proUferum. 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 theilowers, 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- 

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

 structure is generally accompanied with a greater tendency 

 to permanence in form," says Dr. Hooker ; or, conversely, 

 " the least complex are also the most variable." This is the 

 second aspect under which we have to contemplate the facts. 



The dift'erences between the simpler Algw and Fungi, and 

 Oetween them and the Lichens, are so feebly marked that 

 botanists have been unable to frame satisfactory definitions 

 of these classes. " Linnaeus, for instance, and Jussieu, con- 

 sidered Lichens as forming a part of Algce, in which they 

 are followed by Fries." Mr. Berkeley, however, quoting the 

 admission of Fries " that there is no certain distinction be- 

 tween Lichens and Fungi, except the presence in the former 

 of green globules, resembling grains of chlorophyll," him- 

 self prefers to unite Fungi and Lichens under the general 

 head of Mycetales. This structural indefiniteness is accom- 



