196 SECTIONAL ADDRESSES. 



I think, believe with the late Prof. Anstruther Lawson, that hybridisation 

 has played an important part in the production of many of the closely 

 similar forms, which may or may not breed true, and which are so great 

 a puzzle to the botanist. 



In addition to questions relating to possible hybridisation, we are now 

 also recognising the importance of soil factors in connection with our 

 conceptions of species. To me it has always been fascinating, when 

 wandering in the Alps or elsewhere, to note the changes in the vegetation 

 when passing from one geological formation to another, more especially 

 when, as is so often the case, it is a question of the presence or absence 

 of lime. Near Mont Cenis, for instance, the line of demarcation between 

 Gentiana lutea and G. purpurea is as sharp as if a dividing fence were 

 present, and there are equally striking changes with other plants near 

 Le Lautaret, where there are marked changes in the underlying rocks. 

 Examples will readily occur to you, and in all such cases we are dealing 

 with definite and well-marked species, but we may be permitted to speculate 

 whether the allied species, now found restricted to certain types of soil, 

 may not, in times long past, have diverged from a common ancestor. 



It is when we come to more subtle cases, to some of which I shall 

 refer later, where owing perhaps to a change of soil a physiological difference 

 can be detected, without any obvious corresponding morphological change, 

 that our interest becomes acutely aroused, and we realise the depth of our 

 ignorance. Physiological varieties of this nature can sometimes be 

 assumed to be due to the nature of the soil, and in the case of certain 

 plants restricted to the Serpentine rocks, some well-marked morphological 

 characters can also be recognised. 



A somewhat parallel case, though of a different order, is afforded by the 

 common Mistletoe, Viscum album. 



Tubeuf , you will remember, in his monograph on the Mistletoe,'* gives 

 an account of the races or varieties of Viscum album, which are definitely 

 associated with particular host plants. Much has been written about 

 these ' forms,' and they have even been given definite specific names 

 (e.g. Viscum austriacum, Viscum laxum), but for this there does not seem 

 adequate reason. Three definite physiological races, however, are clearly 

 marked, (1) the form which is found on deciduous trees, (2) that associated 

 with the Silver Fir, Abies pectinata and other species of Abies, and (3) the 

 form parasitic on Pinus sylvestris, P. Laricio and P. montana. 



The races are so far distinct that seeds of the ' Pine form,' for instance, 

 will not grow on the apple or fir, and vice versa. Physiologically, therefore, 

 they are distinct, though morphologically they cannot be separated. A 

 case like this suggests that we may be witnessing the advent of three 

 species from one, and that eventually morphological differences may also 

 become evident. 



The vegetation of South Africa supplies some Taxonomic physiological 

 problems of a like nature, which up to the present have not been satis- 

 factorily solved by the Systematic Botanist. These relate to the difficulty 

 of differentiating between two or more forms of the same species which, 

 though distinct physiologically, cannot be separated on any structural 



s ' Monographic der Mistel,' Karl Freiherr von Tubeuf (1923), pp. 661-672. 



