THE DISCAL I'LORETS OF SEXECIO JACOBOEA 



Bv SIR W. W. STKR-KLANI). H.A. 



A GOOD many years ago, I began a series of observa- 

 tions on the phyllotaxis of composite flowers, in the 

 North of Italv. By a curious coincidence. Professor 

 Ludw igof Gratzwas makingverysimilar observations, 

 which he pubhshed about the year 1894. But while 

 he contined himself to the ray florets I have more or 

 less completel}' solved the problem of the discal 

 florets, and the solution, if my observations are ever 

 published, will completely transform our ideas of the 

 evolution of organic form. 



What has been established \\ ith absolute certaiiit\- 

 about the ra\ed florets of the Compositae is that, in 

 the vast majority of cases, where they do not exceed 

 fift\'-five in number — beyond which statistics have 

 not yet probed, thev follow the law of phyllotaxis. 

 In other words, in the vast majoritv of cases the 

 raved florets are three, five, eight, thirteen, twenty- 

 one, thirt\"-four, and perhaps flfty-five in number. 

 Eight and thirteen are also doubled and in the small 

 wild calendula we have a maximum at twenty-six, 

 with the principal maximum at sixteen, and in the 

 common wild blue chicory {Iiifyha cliicarcu) at 

 sixteen with a large sub-maximum at fifteen. A 

 scarce large flowering Achillea found near Sonnico 

 below Edolo — the specific name of which I forget, 

 and, not having my notes with me, consequently 

 cannot give — has also sixteen rays, and doubtless 

 there are manv other instances. With respect to 

 the number twenty-six, I cannot as yet give an 

 explanation, but as regards sixteen there is a good 

 deal to be said. 



Iiifyba cliicorea, as observed, gives a small 

 maximum at sixteen, very nearly reached by the 

 sub-maximum at fifteen, after which there is a large 

 drop, and the reason of it is apparent to the naked 

 eye. Naturally when the seeds set, the fruits retain 

 the position of the florets. In these flowers of Intyha 

 chicorea, the seeds set in the form of a more or less 

 pentagonal tessellated pavement, and in the case of 

 the seed-head with the sixteen constituent parts, the 

 arrangement was also obvious to the naked eye. A 

 ring of five fruits was surrounded b}- a ring of eleven. 

 I then analysed geometricall)' the seed-heads of 

 Intyha chicorea for all numbers from thirteen to 

 twentv-one, and it was demonstrated, with mathe- 

 matical certainty, that in all cases they represented 

 whole or partial concentric rings of circles beginning 

 with a ring of five. Thirteen represents a ring of five 

 surrounded by eight out of the eleven of the second 

 ring, sixteen represents five surrounded by the 

 complete second ring of eleven, twenty-one repre- 

 sents sixteen with five out of the seventeen of the 

 third or next outer ring, and so of all the rest. I 

 need not, perhaps, remark that each concentric ring 

 increases by six circles composing it so that three 

 complete rings of circles sum 5 + 11 + 17 = 33- 



.\s I have said, the reason why in Intyba chicorea 

 we have a maximum of sixteen florets and a sub-maxi- 

 mum of fifteen is obvious at once, because sixteen is 

 the sum of the first and second rings of two concen- 

 tric rings of circles beginning with a ring of five. 

 .•\nother i)otanical fact in connection with these 

 discoveries clenches the argument and throws a 

 flood of light upon the evolution of form. An allied 

 species to Intyha chicorea, but rarer and growing in 

 drier and stonier habitats, is Scariola. It is yellow 

 flowering, often found by rocky beds of torrents 

 dry in summer. Its chief habitat is the lower part 

 of the Val de Sole, a stony valley with vast moraines 

 from the glacial epoch, in that part of the Austrian 

 Tyrol which ought to belong to Italy. Here the 

 flower heads of Scariola are, I may say, invariably 

 composed of eleven florets. Although eleven is 

 practically the invariable number, last autumn I 

 found a few [slants of a dwarfed Scariola, growing at 

 the foot of a high stone wall, flanking the exposed 

 and dusty road from Salo to Madcrno on the Lago 

 di Garda. The spot was dry and ston\-, without 

 grass and w ith hardly anything else but the Scariola 

 growing upon it. A great many of the flower-heads 

 of these plants were composed of only ten florets. 

 Why has Scariola eleven florets ? The reason seems 

 to me clear, \\/.., that when Intyha chicorea, which 

 is found on grassy roadsides, straggled on to stony 

 ground, all its parts shrunk, the florets turned yellow, 

 the stalks grew thinner and lankier and the inner 

 five florets were squeezed out of existence, and only 

 the eleven outer circlet of florets remained. Further 

 contraction caused a re-arrangement visible to the 

 naked eve : three of the eleven forming an inner 

 circle of three, surrounded by a ring of eight : which 

 corresponds to two concentric rings of circles, 

 beginning with a ring of three with one of the outer 

 nine missing. That this is the true explanation can 

 be inferred from other similar facts. Prenanthe 

 mural is is a form of hawkweed though put in 

 another genus, which, as the name implies, is attached 

 to a stonv habitat, and has its florets reduced invari- 

 ably to five in number. A similar Prenanthe, but 

 rarer and with purple flower heads, haunts similar 

 localities, and its flower-heads also consist invariably 

 of onlv five florets. It is abundant about Torno on 

 the Lake of Como, and is remarkable for the fact that 

 it is practicallv impossible to dry it and prevent its 

 florets from turning to fruits in the process. Some- 

 thing of the same kind occurs in the subject of the 

 present essay. When flowers belonging to plants 

 growing on stony ground are analysed, it is found 

 that the ligules of the ra\-ed florets are longer than 

 those of plants growing on relatively rich soil, also 

 the discal florets are much less numerous. Thus in 

 the former case the number of discal florets ranges 



