303 



DIELS'S LAW. 



In the year 1906, Dr. L. Diels published a small book (royal Svo, pp. 130) 

 entitled " Jugendformen ixnd Blutenreife im Pflanzenreich " (Juvenility and flowering 

 ripeness). I have already drawn attention to this work in Part XLIX, p. 273, but 

 think the thesis mainly referred to in it should be dealt with under the name of Diels's 

 Law. I will presently restate the thesis or law. 



It may be convenient, at the present place, to offer a translation of certain 

 passages in Diels's work : — 



(p. S9 of the work.) Eucalyptus Risdoni. J. D. Hooker described E. Risdoni (Hooker's Loudon 

 Journal of Botany, v, 477) in 1847 from Gunn's Southern Tasmanian collections. He characterised the 

 species specially by its foliage " foliis oppositis ovato-cordatis acuminatis, sessilibus, vel basi lata connatis, 

 junioribus ramulis alabastrique pulverio-glaucentibus." The collector's notes give 6 to 7 metres as the 

 height of the tree. From Hooker's description of the generative parts of the tree (see fig. 25 b, c, d, e of 

 the work) one can draw the conclusion that these organs are nearly related to those of E. amygdalina. 



(p. 90.) Though the foliage of the full-grown E. amygdalina is lanceolate and pointed, it resembles 

 closely those of E. Risdoni in their juvenile state. (Fig. 25 a.) (N.B. — No form of E. amygdalina resembles 

 25 a. — J.H.M.) E. amygdalina is widely spread in south-eastern Australia and Tasmania, is that poly- 

 morphous species which attains gigantic dimensions, and is found occasionally over 100 metres high, but 

 generally its height is more moderate, and Bentham describes it from travellers' notes as " A tree usually 

 small or moderate-sized." The number of forms of E. amygdalina are first thoroughly described by Mueller 

 in Fragm. xii (1860). At page 54 we have the following sentence : " Transitus autem claros ab E. Risdoni, 

 qualem J. Hooker depixit, ad illam E. atnygdafincr formam, quam Siebcr sub nomine E. radiatw distribuit, 

 a cl Oldfield accepi." F. v. Mueller repeats this opinion more in detail in his " Eucalyptographia," 1880. 

 He regards E. Risdoni Hook, f., ' ; only as an aberrant form of E. amygdalina. E. Risdoni is to the present 

 day only known from southern Tasmania. It is a small tree. The leaves of the upper branches are mostly 

 thick and stiff, proportionally short and nearly equal-sided, while the leaves of the lower branches are, 

 like those of the seedling plants and adventive shoots, opposite, sessile, broad, often connate, and covered 

 with a whitish powder, also the branchlets and umbels, besides the fruits are generally larger. But these 

 characters are only gradual, and not really specific. Further, Bentham, who saw only dried material, 

 admitted (B. El. iii, 203) that the characters of E. Risdoni were only gradual distinctions, and that " Our 

 dried specimens do not admit of our fixing any precise limits, and in that state it is sometimes scarcely 

 possible to decide to which species they should be referred." These opinions of the most eminent experts 

 make the facts perfectly clear. In the generative characters there is no distinction between E. amygdalina 

 and E. Risdoni, and in the vegetative characters, E. Risdoni appears as the juvenile form of E. amygdalina. 

 Our figure Xo. 25 gives everyone an opportunity of forming his own opinion. The useless trouble some 

 former botanists took to find out some minute distinctions between the two species should be ignored at 

 the present day ! These minute distinctions disappear if compared with the mass of characters they have 

 in common." (If by this Dr. Diels considers the differences between E. Risdoni and E. amygdalina to be 

 minute he will get no modern botanist to agree with him. Many of the demarcations of species as laid 

 down by Mueller have been abandoned as field knowledge has become more accurate. See beloWj 

 p. 305.— J.H.M.) 



(p. 91). The relationship between E. Risdoni and E. amygdalina caused Mueller to point out similar 

 relationships between other species in the genus Eucalyptus. E. pulverulenta and E. melanophloia seem 

 to be in the same position to E. Stuarliana and E. crebra, as E. Risdoni is to E. amygdalina. (Mueller, 

 " Eucalyptographia," under E. amygdalina) 



I p. 92). With these statements Mueller has proved undisputably that a vegetative juvenile form and a 

 vegetative full-groui' form can exist in a single species, and each form flowers and fruits and forms a perfectly 

 dosed cycle of life. (The words in italics are what I propose to call Diels's Law. See below, p. 305. — ■ 

 -l.H.M.) It is quite unlikely that these relations will be found to be confined to our present-day species. 



