Dec. 12, 1878] 



NATURE 



-^T^Z 



tury (1759), in regarding cells as the result of the action of an 

 organising power upon a matrix, and not as themselves influenc- 

 ing organisation, were adverse to the progress of histology. It 

 is from Schleiden (1838), who described the cell as the true unit 

 of vegetable structure, and Schwann, who extended this view 

 to all organisms whether plants or animals, and gave its modern 

 basis to biology by reasserting the unity of organisation 

 throughout animated nature, that we must date the modern 

 achievements of histological science. Seldom, perhaps, in the 

 history of science has any one man been allowed to see so mag- 

 nificent a development of his ideas in the space of his own 

 lifetime as has slowly grown up before the eyes of the venerable 

 Schwann, and it was, therefore, with peculiar pleasure that a 

 letter of congratulation was intrusted by the officers to one of 

 the Fellows of this Society on its behalf on the recent occasion 

 of the celebration of the fortieth anniversary of Schwann's 

 entry into the professoriate. 



If we call up in our mind's eye some vegetable organism and 

 briefly reflect on its construction, we see that we may fix on 

 three great steps in the analysis of its structure, the organic, 

 the microscopic, and the molecular, and, although not in the 

 fame order, each of the three last centuries is identified with 

 one of these. In the seventeenth century Grew achieved the 

 microscopic analysis of plant tissues into their constituent cells ; 

 in the iSth, Caspar Wolff effected the organic analysis (inde- 

 pendently but long subsequently expounded by the poet Goethe) 

 of plant structures into stem and leaf. It remained for Nageli 

 in the present century to first lift the veil from the mysterious 

 processes of plant growth, and by his memorable theory of the 

 molecular constitution of the starch-grain and cell-wall, and 

 their growth by intussusception (1858), to bring a large class of 

 vital phenomena within the limits of physical interpretation. 

 Strasburger has lately (1876) followed Sachs in extending 

 Nageli's views to the constitution of protoplasm itself, and there 

 is now reason to believe that the ultimate structure of plants 

 consists imiversally of sol'.d molecules (not, however, identical 

 with chemical molecules) surrounded with areas of water which 

 may be extended or diminished. While the molecules of all 

 the inert parts of plants (starch-grains, cell-wall, &c.) are on 

 optical grounds believed to have a definite crystalline character, 

 no such conclusion can be arrived at with respect to the mole- 

 cules of protoplasm. In these molecules the characteristic pro- 

 perties of the protoplasm reside, and are more marked in the 

 ^ggi^^^te mass in proportion to its denseness, and this is due 

 to the close approximation of the molecules and the tenuity of 

 their water)' envelopes. The more voluminous the envelopes 

 the more the properties of protoplasm merge in those of all 

 other fluids. 



It is, however, to the study of the nuclei of cells that attention 

 has been recently paid with the most interesting results. These 

 well-known structures, first observed by Ferdinand Bauer at the 

 beginning of the century (1802), were only accurately described, 

 thirty years later, by Robert Brown (1833). Up to the present 

 time their function has been extremely obscure. The beautiful 

 investigations of Strasburger (1875) have led him to the con- 

 clusion that the nucleus is the seat of a central force which has 

 a kind of polarising influence upon the protoplasm molecules, 

 causing them to arrange themselves in lines radiating outwards. 

 Cell-division he regards as primarily caused by the nucleus be- 

 coming bipolar, and the so-called caryolitic figures first described 

 by Auerbach exhibit the same arrangement of the protoplasm 

 molecules in connecting curves as in the case of iron-filings about 

 the tB'o poles of a bar-magnet. The two new centres mutually 

 retire, and each influencing its own tract of protoplasm, the cell- 

 division is thereby ultimately effected. This is but a brief 

 account of processes which are greatly compl cated in actual 

 detail, and of w hich it must be remarked that, while the interest 

 and beauty of the researches are beyond question, caution must 

 be exercised in receiving the mechanical speculations by which 

 Strasburger attempts to ex];Iain them. He has him.self shown 

 that cell-division presents the same phenomena in the animal 

 kingdom, a result which has been confirmed by numerous 

 observers, amongst whom I may content myself with mention- 

 mg one of our own number, Mr. F. Balfour, Strasburger further 

 pomts out that this affords an argument for the community of 

 descent m animal and vegetable cells; he regards free cell- 

 dmsion as denrable from ordinary cell-division by the suppre- 

 aon of certain stages. 



_ Turning now to the discoveries made during the last five years 

 in physiological botany, we find that no one has advanced this 



subject so greatly as Mr. Darwin, In 1875 was published his 

 work on insectivorous plants, in which he ascertained the fact 

 that a number of species having elaborate structures adapted for 

 the capture of insects, utilised the nitrogenous matter which these 

 contain as food. The most important principle established ia 

 the course of these researches was that such plants as Drosera, 

 Dioncea, Pirtguicula, &.C., secrete a digestive fluid, which has led, 

 through Gorup Bezanez's investigations on the ferment in ger- 

 minating seeds, to a recognition of the active agency of ferments 

 in the transmission of food -material, which marks a great advance 

 in our knowledge of the general physiology of nutrition. 



The extreme sensitiveness of the glands of Drosera to 

 mechanical and chemical stimulus (especially to phosphate of 

 ammonia), the directive power of its tentacles, depending upon 

 the accurate transmission of motor impulses, and the ' ' reflex " 

 excitation of secretion in the glands, were all discoveries of the 

 most suggestive nature in connection with the subject of the 

 irritability and movements of plants. The phenomenon of the 

 aggregation of the protoplasmic cell-contents in the tentacles of 

 Drosera is a discovery of a highly remarkable nature, though 

 not yet thoroughly understood. Lastly, Mr. Frank Darwin, 

 following his father's footsteps, as it were crowned the edifice by 

 sho%ving to what an extent insectivorous plants do profit by 

 nitrogenous matter supplied to their leaves. 



In close relation to these researches are those, also by Mr. 

 Darwin, on the structure and functions of the bladder of Utri- 

 cularia, which he has shown to have the power of absorbing de- 

 caying animal matter ; and those of Mr. Frank Darwin on con- 

 tractile filaments of extraordinary tenuity attached to the glands 

 on the inner surface of the cups formed by the connate bases of 

 the leaves of the teasel, and which filaments exhibit motions 

 suggesting a protoplasmic origin. It is to be hoped that their 

 discoverer will pursue his investigations upon these curious 

 bodies, whose origin and real nature in relation to the plant and 

 its functions is involved in obscurity. 



The subject of the cross-fertilisation of plants, which, though 

 a long-known phenomenon, fir.-t become a fruitful scientific study 

 in Mr. Darwin's now classical work, " On the Various Contri- 

 vances by which Orchids are Fertilised," has within the last few 

 years made rapid advance imder its author's hand. The extreme 

 importance of avoiding self- fertilisation might indeed be inferred 

 from the prevalence in flowers of elaborate contrivances for pre- 

 venting it ; but it remained to be shown that direct benefit 

 attended cross-fertilisation, and this has now been proved by an 

 elaborate series of experiments, the results of which are not 

 only that both increased fertility or greater vigour of constitution 

 attend cross-fertilisation, but that the opposite effects attend self- 

 fertilisation. In the course of these experiments it became 

 evident that the gcod effects of the cross do not depend on the 

 mere fact of the parents being different individuals, for when 

 these were grown together and under the same conditions, no 

 advantage w as gained by the progeny ; but when grown under 

 different conditions a manifest advantage was gained. As 

 instances, if plants of Ipomaa and Mimulus, which had been 

 self-fertilised for seven previous generations, were kept together 

 and then intercrossed, their offspring did not profit in the least ; 

 ^^hereas, when the parent plants were grown under different 

 conditions, a remarkably vigorous offspring was obtained. 



Mr. Darwin's last work, "On the Different Forms of 

 Flowers," though professedly a reprint of his paper on dimorphic 

 plants, published by the Linnean Society, contains many addi- 

 tions and new matter of great importance in reference to the 

 behaviour of polygamous plants, and en cleistogamic flowers. 

 Among other points of great interest is the establishment of very 

 close analogies between the phenomena attending the illegitimate 

 union of trimorphic plants and the results of crosses between 

 distinct species : the sterile offspring of the crosses of the same 

 species exhibiting the closest resemblance to the sterile hybrids 

 obtained by crossing distinct species ; while a w hole series of 

 generalisations, founded on the results of the one series of ex- 

 periments, are closely pai-alleled by those founded on the other. 

 The bearing of this analogy on the origin of species is obviously 

 important. 



Besides these investigations, Mr, Darwin has produced within 

 the last five years second editions of his volume on the ' * Ferti- 

 lisation of Orchids," and on the "Habits and Movements of 

 Climbing Plants," as also of his early works on " Coral Reefs," 

 and " Geological Observations in South America ; " all of them 

 abounding in new matter. 



Of special interest to myself, as having been conducted in the 



