540 



NATURE 



[September 27, 1900 



the systematic position of various genera, and sometimes of 

 larger groups. Thus the CycadaceDe have been removed from 

 among the Monocotyledons, and the Conifera; from among the 

 Dicotyledons, where de CandoUe placed them, and have been 

 united with the Gnetacere into the sub-class Gymnospermje. 

 The investigation of the development of the flower, in which 

 Payer led the way, and the elaboration of the floral diagram 

 which we owe to Eichler, have done much, though by no means 

 all, to determine the affinities of doubtful Angiosperms, especi- 

 ally among those previously relegated to the lumber-room of the 

 Apetalre. 



Anatomy and Histology. 

 Passing now to the consideration of the progress of knowledge 

 concerning the structure of plants, the most important result to 

 be chronicled is the discovery that the plant-body consists of 

 living substance indistinguishable from that of which the body of 

 animals is composed. The earlier anatomists, whilst recognising 

 the cellular structure of plants, had confined their attention to 

 the examination of the cell-walls, and described the contents as 

 a watery or mucilaginous sap, without determining where or 

 what was the seat of life. In 1831 Robert Brown discovered the 

 nucleus of the cell, but there is no evidence that he regarded it 

 as living. It was not until the renascence of research in the 

 'forties, to which I have already alluded, that any real progress 

 in this direction was made. The cell-contents were especially 

 studied by Naegeli and by Mohl, both of whom recognised the 

 existence of a viscous substance lining the wall of all living cells 

 as a "mucous layer" or "primordial utricle," but differing chemi- 

 cally from the substance of the wall by being nitrogenous : this 

 they regarded as the living part of the cell, and to it Mohl (1846) 

 gave the name "protoplasm," which it still bears. The full 

 significance of this discovery became apparent in a somewhat 

 roundabout way. Dujardin, in 1835, h^d described a number 

 of lowly organisms, which he termed Infusoria, as consisting of 

 a living substance, which he called " sarcode." Fifteen years 

 later, in a remarkable paper on Protococcus phivialis, Cohn drew 

 attention to the similarity in properties between the " sarcode " 

 of the Infusoria and the living substance of this plant, and 

 arrived at the brilliant generalisation that the " protoplasm " of 

 the botanists and the "sarcode" of the zoologists are identical. 

 Thus arose the great conception of the essential unity of life in 

 all living things, which, thanks to the subsequent labours of such 

 men as de Bary, Briicke, and Max Schultze, in the first instance, 

 has become a fundamental canon of Biology. 



A conspicious monument of this period of activity is the 

 cell-theory propounded by Schwann in 1839. Briefly stated, 

 Schwann's theory was that all living bodies are built up of 

 structural units which are the cells : each cell possesses an in- 

 -dependent vitality, so that nutrition and growth are referable, 

 not to the organism as a whole, but to the individual cells. 

 This conception of the structure of plants was accepted for many 

 years, but it has had to give way before the advance of ana- 

 tomical knowledge. The recognition of cell-division as the 

 process by which the cells are multiplied — in opposition to the 

 Schleidenian theory of free cell-formation — early suggested 

 doubts as to the propriety of regarding the body as being built 

 up of cells as a wall is built of bricks. Later the minute study 

 of the Thallophyta revealed the existence of a number of plants, 

 such as the Myxomycetes, the phycomycetous Fungi, and the 

 siphonaceous Alga;, some of them highly organised, the vege- 

 tative body of which does not consist of cells. It became clear 

 that cellular structure is not essential to life ; that it may be 

 altogether absent or present in various degree. Thus in the 

 higher plants the protoplasm is segmented or septated by walls 

 into uninucleate units or "energids" (Sachs), and such plants 

 are well described as "completely septate." But in others, 

 such as the higher Fungi and certain Algte (e.g. Cladophora, 

 Hydrodictyon), the protoplasm is septated, not into energids, 

 but into groups of energids, so that the body is " incompletely 

 septate." Finally there are the Thallophyta already enu- 

 merated, in which there is complete continuity of the proto- 

 plasm : these are "unseptate." Moreover, even when the 

 body presents the most complete cellular structure, the energids 

 are not isolated, but are connected by delicate protoplasmic 

 fibrils traversing the intervening walls ; a fact which is one of 

 the most striking discoveries in the department of histology. 

 This was first recognised in the sieve-tubes by Ilartig (1837) ; 

 then by Naegeli (1846) in the tissues of the Florideae. After a 

 long period of neglect the matter was taken up once more by 



NO. 1613, VOL. 62] 



Tangl (1880), when it attracted the attention of many investi- 

 gators, as the result of Whose labours, especially those of Mr. 

 Gardiner, the general and perhaps universal continuity of the 

 protoplasm in cellular plants has been established. Hence the 

 body is no longer regarded as an aggregate of cells, but as a 

 more or less septated mass of protoplasm : the synthetic 

 standpoint of Schwann has been replaced by one as distinctively 

 analytic. 



Time does not permit me to do more than mention the im- 

 portant discoveries made of late years, mainly on the initiative 

 of Strasburger, with regard to the details of cytology, and 

 especially to the structure of the nucleus and the intricate dance 

 of the chromosomes in karyokinesis. Indeed, I can do but scant 

 justice to those anatomical discoveries which are of more 

 exclusively botanical interest. One important generalisation 

 which may be drawn is that the histological diff"erentiation of the 

 plant proceeds, not in the protoplasm, as in the animal, but in 

 the cell-wall. It is remarkable, on the one hard, how similar 

 the protoplasm is, not only in diff"erent parts of the same body, 

 but in plants of widely different affinities ; and, on the other, 

 what diversity the cell-wall offers in thickness, chemical com- 

 position, and physical properties. In studying the differentia- 

 tion of the cell-wall the botanist has received valuable aid from 

 the chemist. Research in this direction may, in fact, be said to 

 have begun with Payen's fundamental discovery (1844) that the 

 characteristic and primary chemical constituents of the cell-wall 

 is the carbohydrate which he termed cellulose. 



The amount of detailed knowledge as to the anatomy of plants 

 which has been accumulated during the century by countless 

 workers, among whom Mohl, Naegeli, linger and Sanio 

 deserve special mention as pioneers, is very great — so great, 

 indeed, that it seemed as if it must remain a mere mass of facts 

 in the absence of any recognisable general principles which 

 might serve to marshal the facts into a science. The first step 

 towards a morphology of the tissues was Hanstein's investiga- 

 tion of the growing point of the Phanerogams (1868), and his 

 recognition therein of the three embryonic tissue-systems. This 

 has lately been further developed by the promulgation of van 

 Tieghem's theory of the stele, which is merely the logical out- 

 come of Hanstein's distinction of the plerome. It has thus become 

 possible to determine the homologies of the tissue-systems in 

 different plants and to organise the facts of structure into a 

 scientific comparative anatomy. It has become apparent that, 

 in many cases, differences of structure are immediately traceable 

 to the influence of the environment ; in fact, the study of 

 physiological or adaptive anatomy is now a large and important 

 branch of the sut^ject. 



The study of Anatomy has contributed in some degree to the 

 progress of systematic Botany. It is true that some of the more 

 ambitious attempts to base classification on Anatomy have not 

 been successful ; such, for instance, as de Candolle's subdivision 

 of Phanerogams into Exogens and Endogens, or the subdivision 

 of Cormophyta into Acrobrya, Amphibrya, and Acramphibrya, 

 proposed by Unger and Endlicher. Still it cannot be denied 

 that anatomical characters have been found useful, if not abso- 

 lutely conclusive, in suggesting affinities, especially in the 

 determination of fossil remains. A large proportion of our 

 knowledge of extinct plants, to which I have already alluded, 

 is based solely upon the anatomical structure of the vegetative 

 organs ; and although affinities inferred from such evidence 

 cannot be regarded as final, they suffice for a provisional 

 classification until they are confirmed or disproved by the 

 discovery and investigation of the reproductive organs. 



Physiology. 

 The last branch of botanical science which I propose to pass 

 in review is that of physiology. We may well begin with the 

 nutritive processes. At the close of the eighteenth century 

 there was practically no coherent theory of nutrition ; such as 

 it was it amounted to little more than the conclusion arrived at 

 by van Helmont a century and a half earlier, that plants require 

 only water for their food, and are able to form from it all the 

 different constituents of their bodies. It is true that the im- 

 portant discovery had been made and pursued by Priestley 

 (1772), Ingen-Housz (1780) and Senebier (1782) that green 

 plants exposed to light absorb carbon dioxide and evolve free 

 oxygen ; but this gaseous interchange had not been shown to be 

 the expression of a nutritive process. At the opening of the 

 nineteenth century (1804) this connection was established by 

 de Saussure, in his classical " Recherches Chimiques," who 



