NOTES 711 



against intense illumination, prevention of rapid change of temperature, etc." 



Warburg evidently puts forward this counsel of perfection with the intention 

 of showing that the conception of a "dermal tissue" is inadmissible; but he has 

 overlooked the fact that a dermal tissue does not perform St sral principal functions, 

 but, on the contrary, a single principal function which comprises several partial 

 functions. 



28. G. Haberlandt : Eine botanische Tropenreise, Leipzig, 1893, p. His. Stahl : 

 Ann. J3uit. 11, 1893 (rainfall and leaf-structure). 



29. A discussion of the various ways in which the different tissues and organs of 

 the plant-body are brought into harmonious cooperation with one another would 

 be outside the province of this book. Very suggestive remarks on this subject will 

 be found in Roux's Kampf der Theile im Organismus, Leipzig, 1881 (second extended 

 version in Ges. Abh. 1, 1895, pp. 135-422). Weisniann also enters into the question 

 in his stimulating essay on Aussere Einfiusse als Entwickelungsreize, Jena, 

 1894. 



.'50. Since the publication of Schwendener's Mechanische Prinzip, and G. Haber- 

 landt 's Entwickelungsgeschichte des mechanischen Gewebesystems, the theo- 

 retical and practical value of the anatomico-physiological classification of tissues 

 has repeatedly been the subject of vigorous discussion. A few words must be devoted 

 to this controversy at the present stage. 



That the anatomico-physiological classification is sound in principle, is now 

 generally admitted. De Bary's Comparative Anatomy, though purely descrip- 

 tive in character, lays stress upon the correlation between structure and func- 

 tion. Thus, on p. 2 of the introduction we find the following remarks: 

 "" Investigation shows that the adaptation to, and participation in vegetative 

 duties, that is, the development hito organs of definite function, and corresponding' 

 structure, is far the most commonly and definitely carried out for members of 

 lower ranks, i.e. for cells, and groups of cells, or the products of their metamorphosis." 

 De Bary accordingly (I.e. pp. 24 sqq.) always treats the structure and arrangement 

 of tissues from the point of view of adaptation. He further distinguishes between 

 obvious adaptations, on the one hand, and unexplained anatomical features, on the 

 other ; the latter are also " derived from adaptations, winch have happened in some 

 epoch or other of the phylogenetic development, but which cannot now be certainly 

 referred to their causes." In pointing out (I.e. p. 25) the desirability of arranging 

 the various chapters of his book, either according to different aspects of adaptation, 

 or else with reference to taxonomic groups, he recognises that the anatomico- 

 physiological classification is not only legitimate but also eminently practicable. 

 In fact, it was only the existence of a large number of unexplained anatomical 

 features, that led him to prefer a purely descriptive treatment. A considerable 

 time has elapsed since De Bary's book was written (it was begun in 1865), and 

 the unexplained characters are no longer so numerous as to place a serious obstacle 

 in the way of a rational anatomico-physiological classification of tissues. 



It is interesting to consider what attitude one of the most noted of plant-physio- 

 logists, Sachs, has taken up with regard to the anatomico-physiological classification 

 of tissues. In the introduction to the eighth chapter of his last great work (Lectures 

 on Plant-Physiology) he defines tissues as '"masses of cells, which in their growth 

 and other physiological relations, present a certain agreement, and are distinguished 

 from the other neighbouring masses of tissue." Similarly, a tissue-system is defined 

 as "' a union of tissues " which " constitute a whole of definite physiological character." 

 In view of these statements it might have been expected that Sachs would have put 

 the anatomico-physiological system of classification into practice forthwith. As a 



