ERIGERON MUCRONATUM. POINTED ERIGERON. 75 



Each floret in a compound flower is a metamorphosed 

 branchlet with its stem and leaves. The corolla is probably 

 formed of five primordial leaves ; generally united into a tube 

 (Fig. 3) in the central or disc flowers, but bursting on one side 

 and looking like a single strap in the ray florets. The peculiar 

 force which accomplishes this rolling up process in the formation 

 of the tube, or in opening the tube to make a strap-shaped 

 floret, whichever it may be, varies in different genera. Usually, 

 the outer row alone is strap-shaped and the rest all tubular, 

 or all may be strap-shaped, or the whole tubular. But a 

 peculiarity of this Erigcron is that there are two series of strap- 

 shaped corollas, while all the rest are tubular as in Fig. 3. 



But in some species of plants there are not only transitions 

 of leaves, but transitions in branches, and it is one of the fortu- 

 nate features of this species that it affords the illustration. 

 Some species of Erigerou have a matted root-stock ; others send 

 out thread-like stems, with perhaps a few very small scales, 

 terminating in a bud which eventually becomes a young plant, 

 as in the runner of a strawberry. In the case of the strawberry 

 there are instances known where the runner erects itself and 

 becomes a bunch of flowers with leaves, and finally fruit. We 

 thus know that the thready runner and the flowering branch are 

 essentially the same thing, but, under the action of some peculiar 

 phase of growth force, one has changed its manner and form for 

 the other. In our present species we have, in Fig. 2, a branch 

 just intermediate between its own flower stems, and the thready 

 runners or stolons of other species ; and we see how readily the 

 one may be transformed into the other. 



Now these peculiar gradations which we find in the individual, 

 and the science of which we know as morphology, exist in the 

 same degree between species, and which then constitute what 

 we call the science of evolution. In these Erigci^ons it is very 

 well marked. Taking three closely related American species, E. 

 bellidifoliwn, E. }mLcro7iatiim and iS'. Phil add phicimi, we have thc! 

 last with matted root stocks, the first with lonfj thready stolons, 



