INTRODUCTION. 



3. INTEBMEDIATENESS AND LESSENED VITALITY 

 OF HYBRIDS, ETC. 



The gross structural characters of plants have at- 

 tracted the attention of mankind from time immemorial, 

 and for generations they have constituted the essential 

 means by which plants have been differentiated and 

 classified; yet beneath them there lay an infinitude of 

 microscopical, chemical, physical, and physico-chemical 

 properties of tissues and various protoplasmic substances 

 which will undoubtedly be found to be of far greater sig- 

 nificance in differentiation, not only in taxonomy and 

 phylogeny, but also in the elucidation of various prob- 

 lems that constantly confront the botanist. The scien- 

 tific value of the histological method of plant study to 

 the systematist was satisfactorily demonstrated in 1883 by 

 Radlkofer in " Tiber die Methoden in der botanischen 

 Systematik insbesonderedieanatomische Methode." This 

 method he holds is applicable to the study of species, and 

 since his time it has been successfully extended to varie- 

 ties and hybrids. A century ago De Candolle found the 

 microscope useful in plant classification, and Radlkofer 

 predicted in his memoir that the energies of the systemat- 

 ist would for the next century be devoted to the histo- 

 logical method. Previous to the investigations of the 

 latter, much work on the micro-anatomical and the micro- 

 chemical peculiarities of plants was recorded, and since 

 then literature of this character has accumulated to an 

 enormous volume, as is evident at a glance through the 

 encyclopedic pages of Solereder's " Systematische Ana- 

 tomie der Dicotyledonen " that appeared in 1898. While 

 such researches have proved to be of value in taxonomy, 

 in the explanation of many problems that baffled the old- 

 school systematist, and in throwing open new avenues of 

 thought and investigation, but little has been systema- 

 tized that seems to be of immediate practical usefulness 

 to the plant-breeder and to the student of evolution. 

 Time will undoubtedly show, with the sifting out of these 

 records in conjunction with recent work, a wealth of 

 material that far exceeds in value even the greatest 

 expectations. 



All of our knowledge of hybrids dates from a period 

 scarcely more than two centuries ago. It was near the 

 end of the seventeenth century when the existence of 

 sexual organs of plants was recognized, and it was some- 

 time shortly antedating 1719 that Thomas Fairchild, a 

 London gardener, produced a hybrid (Fairchild Sweet 

 William) by the fertilization of Dianthus caryophyllus 

 (the clove pink) with D. barbatus (the common Sweet 

 William). This was followed by investigations of 

 parents and hybrids by Linnaeus. To Kolreuter, how- 

 ever, whose laborious experiments in hybridization began 

 in 1760 by crossing Nicotiana rustica with N. panicw- 

 lata, must be given the credit for laying a working foun- 

 dation that has proved of the greatest value in arousing 

 interest and active investigation in this exceptionally 

 important field of research. What had been recorded 

 of both naturally and artificially produced hybrids up 



to 35 years ago was summarized and commented upon 

 by Focke (Die Pflanzen-Mischlinge : ein Beitrag zur Bio- 

 logie der Gewiichse, 1881). Probably as many as 2,000 

 hybrids are here referred to. Since then the number has 

 been considerably added to in botanical literature. Such 

 investigations, up to the time of the appearance of the 

 memoir by Macfarlane on " A Comparison of the Minute 

 Structure of Plant Hybrids with that of their Parents, 

 and its Bearing on Biological Problems " that appeared 

 in 1892, were confined practically wholly to the grosser 

 phenomena of plant life, such as the parentage, size, 

 vigor, rapidity of growth, length of life, appearance of 

 malformations, fertility, etc. in a word, gross charac- 

 ters such as have been and continue to be the tools of 

 the old-school systematist. 



INTEEMEDIATENESS OF HISTOLOGIC PROPERTIES 

 OF HYBRIDS. 



Macfarlane in referring to the earlier microscopical 

 investigations states that Henslow (Cambridge Phil. 

 Trans., 1831) made a microscopic comparison of a hybrid 

 Digitalis with its parents and showed that in the size 

 and shape of the hairs and other structures the hybrid is 

 intermediate between the parents; that Wichura (Bas- 

 tardefruchtung, 1865) with Salix, and Kerner (Mono- 

 graphia Pulmonar., 1878) with Pulmonaria, likewise 

 found the hybrid to be intermediate ; and that Wettstein 

 (Sitz. der. Kaiser. Akad. der Wissen., 1888), in compar- 

 ing the leaves of four coniferous hybrids observed in 

 transverse sections of the leaves that each hybrid in the 

 number of stomata, depth of the epidermal cells, and 

 number and arrangement of the sclerenchyma elements 

 of the bundles is exactly intermediate between their 

 parents. 



In investigations of the minute characters of over 60 

 hybrids in comparison with their parents, Macfarlane 

 found it necessary to adopt certain precautionary meas- 

 ures in order to secure safe comparative results. Inas- 

 much as they have served as our guide in the anatomical 

 part of the present research they are here quoted in full : 



1. AVEBAOE OBOANISMAL DEVELOPMENT AND DEVIATIONS. 



" It is now recognized by botanists that every species 

 exhibits a sum-total of naked-eye characters which dis- 

 tinguish it with greater or less precision from allied 

 species. These are duly given in every local Flora. 

 But further, specific features alike macroscopic and 

 microscopic which are of great importance, are passed 

 over. Radlkofer (Akad. der Wissenschaften, Munich, 

 1883) has already insisted that the anatomical method 

 must be applied to the study of species, and I have 

 pointed out that this is equally true of subspecies and 

 varieties (Trans. Bot. Soc. Edin., vol. xix, 1891). But 

 it is the sum-total or accumulation of minute peculiari- 

 ties which gives specific identity to any organism, and it 

 is to be expected that evident or naked-eye variations 

 will often have their commencement in trivial structural 

 deviations, which, being perpetuated and exaggerated 

 it may be in size, will ultimately appeal to the naked 



