September 21, 1905] 



NA TURE 



523 



the significance of such nuclear re-arrangements, and if 

 any of them can be definitely explained in this way the 

 final solution of the problem may be much simplified. 



Validity of Cell Structure as seen in Fixed and Stained 

 Preparations. 

 Our knowledge of the minute details of cell structure 

 and nuclear differentiation depends upon the appearances 

 presented by cells which have been fixed in various re- 

 agents and subsequently stained, and it is not an easy 

 matter to determine in how far these are artificial and 

 in how far they are actual structures existing in the living 

 cell. The researches of Fischer, Hardy, Mann, and others 

 have shown that on the precipitation of proteids by re- 

 agents structures are produced which were certainly not 

 present originally, and which resemble those often observed 

 in fixed cells. From a consideration of such facts it has 

 been suggested that many of the details revealed in fixed 

 cells, such as centrosomes and centrospheres, with their 

 fibrillar radiations, are produced artificially and have no 

 real existence. It is unfortunate that so little attention 

 has been paid to the examination of living cells, for the 

 structures which can be seen in them are, so far as they 

 can be revealed by the microscope, always like those seen 

 in fixed preparations. 



Differentiation of Structure Visible in the Living Cell. 



The amount of differentiation visible in the living cell 

 in favourable objects is very considerable. Not only can 

 chloroplasts, starch-grains, nucleus, leucoplasts, pyrenoids, 

 &c., be clearly seen, but also a very considerable amount 

 of detailed structure. Chromosomes have been seen in the 

 living cell by many observers — Treub, Strasburger, 

 Behrens, Zacharias, and others. The series of figures 

 published by Strasburger of nuclear division in the staminal 

 hairs of Tradescantia show the whole process of chromo- 

 some formation and separation into two daughter-groups, 

 except the longitudinal division. 



In the same object Demoor and de Wildeman have also 

 been able to detect the spindle fibres and connecting fibres. 

 These were not seen by Strasburger ; and Zacharias, who 

 has more recently made observations on staminal hairs, 

 was also not able to detect them. Nevertheless Stras- 

 burger mentions that in some cases connecting threads 

 were visible at a late stage in the division between the 

 daughter-nuclei, and Treub also describes a similar pheno- 

 menon in some cases during the nuclear division in the 

 ovules of an orchid. 



In Spirogyra, Strasburger has given a full account of 

 nuclear division in the living cell. Large species of this 

 alga are very favourable objects for this work, and he 

 has shown that in such species the spindle figure as well 

 as the connecting fibres can be seen in the living cell. 

 Wildeman has also seen and figured them ; but Behrens 

 states that spindle fibres and connecting threads are not 

 visible in Spirogyra during life. 



My own observations upon a large species of Spirogyra 

 which I have had an opportunity of investigating entirely 

 support the view that these structures are visible in the 

 living condition. 



The Structure of the Chloroplast. 



In view of its extreme importance in the function of 

 assimilation a knowledge of the structure of the chloro- 

 plast is important. Owing to its small size a satisfactory 

 demonstration of its finer structure is very difficult. That 

 it consists of a colourless ground substance, in which 

 the chlorophyll is embedded, is clear ; but how these two 

 substances are united and the relations between them 

 structurally are not known. Pringsheim concluded that 

 the ground substance of the chloroplast is a sponge-like 

 network with the oil-like solution of chlorophyll in its 

 meshes. 



Schmitz thought that the fine granular appearance of 

 the chloroplast was due to a fine net-like structure in 

 which the chlorophyll was diffused. Fromman also describes 

 it as a green granular network. Schwartz, on the other 

 hand, describes it as composed of a ground substance 

 containing a number of green fibrillse side by side, which 

 are coloured green throughout, but show also an accumu- 



NO. 1873, VOL. 72] 



lation of the green colouring matter in the form of 

 granules along these threads. 



Meyer thought it was composed of a homogeneous 

 ground substance with various-sized granules of the green 

 substance embedded in it. To these granules he gave 

 the name of "grana." Schimper stated that it was com- 

 posed of a colourless stroma containing numerous vacuoles 

 filled with the green semi-fluid chlorophyll, identical with 

 the " grana " of Meyer. 



Some observers consider that the chloroplast is sur- 

 rounded by a distinct membrane ; whilst others consider 

 that the substance of the chloroplast is directly connected 

 by colourless strands to the cytoplasm. 



According to some observations which I have made 

 recently, the chloroplast, when examined under high 

 powers in the living condition, appears to be filled with a 

 mass of green granules with a colourless substance between 

 them. But in certain cases a distinct fibrillar arrange- 

 ment of the chlorophyll is observed. This is very easily 

 seen in the chloroplasts of Euglena, both in the living 

 condition, and, more easily, when the cells are burst and 

 the chlorophyll grains are extruded into the water. But 

 it may be seen also in the chloroplasts of the higher plants 

 when these are large enough to be examined easily. In 

 these cases the green colouring matter appears granular 

 when the chloroplast is in the epistrophe or shade position, 

 fibrillar when it is in the apostrophe or intense light 

 position. This difference in the appearance of the chloro- 

 phyll accompanies a difference in the shape of the chloro- 

 plast. As is well known, the chloroplast in the epistrophe 

 position presents an oval or more or less circular form ; 

 in the apostrophe position a flattened and lenticular form. 

 The fibrillar structure appears to be that of fine fibrils 

 lying more or less parallel, but a closer examination shows 

 that they are connected together here and there so as to 

 give the impression of an elongate network. In the epis- 

 trophe condition the chlorophyll corpuscle appears greener 

 than in the apostrophe condition. The granules are in fact 

 so arranged and so numerous as to present a practically 

 continuous surface of chlorophyll to the action of the light 

 rays. The fibrillar arrangement, on the contrary, has 

 numerous light spaces between the fibrils, so that less 

 surface of chlorophyll is exposed to the rays of light. 

 The difference in the amount of chlorophyll surface exposed 

 to the light appears therefore to be bound up with the 

 difference in the intensity of light which causes the different 

 positions of epistrophe and apostrophe to be assumed by 

 the chloroplast. Just as in diffuse light the chloroplasts 

 themselves are more fully exposed to the light than in 

 intense light, so in the individual chloroplast we appear 

 to have such an arrangement of the chlorophyll that in 

 diffuse light a larger surface of it is exposed to the light 

 rays than in a more intense light. The interesting con- 

 clusion is therefore arrived at, that the chloroplast is 

 able, not only by its position but also by its structure, to 

 guard itself against the effects of a too intense light. 



A careful examination of the chloroplast in the epis- 

 trophe position renders it probable that the granular 

 appearance is not due to the existence of separate granules 

 of chlorophyll. It resembles more nearly an optical effect, 

 due to the superposition of alveoli upon one another, such 

 as appears in fine cil-foams. By focusing carefully above 

 and below the granules we get a distinct appearance as 

 of a green alveolar network. If the chlorophyll corpuscle 

 is extruded into water it begins to swell up and becomes 

 vacuolar ; the granules disappear and the chlorophyll then 

 appears to be distinctly diffused through the ground sub- 

 stance of the chloroplast. I am therefore inclined to the 

 view that the chlorophyll corpuscle consists of a ground 

 substance in the form of a delicate alveolate structure, in 

 which the chlorophyll is more or less uniformly diffused. 

 The diameter of the threads of this network is greater 

 in the epistrophe than in the apostrophe position, and this 

 affords a means by which the chloroplast can accommodate 

 itself to varying intensities of light. 



The chloroplast must be regarded as performing at least 

 two functions. It brings about the dissociation of CO. 

 and it is a starch-forming organ. In the algae and some 

 other plants these two functions appear to be differentiated, 

 and starch is formed directly by the pyrenoid. How far 

 these two functions are independent in the ordinary chloro- 



