390 



NATURE 



\August 21, 1879 



a plane at right angles to a line connecting the two nuclei. The 

 division takes place without the formation of a cell plate such as 

 we saw in the division of the plant cell, and is introduced by a 

 constriction of its protoplasm, which commences at the circum- 

 ference just within the vitelline membrane, and extending towards 

 the centre, divides the whole mass of protoplasm into two halves, 

 each including within it one of the new nuclei. Thus the simple 

 cell which constituted the condition of the egg at the commence- 

 ment of development becomes divided into two similar cells. 

 This forms the first stage of cleavage. Each of these two young 

 cells divides in its turn in a direction at right angles to the first 

 division-plane, while by continued repetition of the same act the 

 whole of the protoplasm or yolk becomes broken up into a vast 

 multitude of cells, and the unicellular organism — the egg, with 

 which we began our history — has become converted into an 

 organism composed of many thousands of cells. This is one of 

 the most widely distributed phenomena of the organic world. 

 It is called "the cleavage of the egg," and consists essentially 

 in the production, by division, of successive broods of cells from 

 a single ancestral cell — the egg. 



It is no part of my purpose to carry on the phenomena of 

 development further than this. Such of my hearers as may 

 desire to become acfiuainted with the further history of the 

 embryo, I would refer to the excellent address delivered two 

 years ago at the Plymouth meeting of the Association by one of 

 my predecessors in this chair — Prof. Allen Thompson. 



That protoplasm, however, may present a phenomenon the 

 reverse of that in which a simple cell becomes multiplied into 

 many, is shown by a phenomenon already referred to — the pro- 

 duction 'of Plasmodia in the Myxomycette by the fusion into 

 one another of cells originally distinct. 



The genus Myriothcla will afford another example in which 

 the formation of plasmodia becomes introduced into the cycle 

 of development. The primitive eggs are here, as elsewhere, 

 true cells with nucleolated nuclei, but without any boundary 

 membrane. They are formed in considerable numbers, but 

 remain only for a short time separate and distinct. After this 

 they begin to exhibit amoeboid changes of shape, project pseudo- 

 podial prolongations which coalesce with those of others in their 

 vicinity, and finally a multitude of these primitive ova become 

 fused together into a common Plasmodium, in which, as in the 

 simple egg cell of other animals, the phenomena of develop- 

 ment take place. 



In many of the lower plants a very similar coalescence is known 

 to take place between the protoplasmic bodies of separate cells, 

 and constitutes the phenomenon of conjugation. Spirogyra is a 

 genus of Alga;, consisting of long green threads common in 

 ponds. Every thread is composed of a series of cylindrical 

 chambers of transparent cellulose placed end to end, each con- 

 taining a sac of protoplasm with a large quantity of cell sap, 

 and with a green band of chlorophyll wound spirally on its 

 walls. When the threads have attained their full growth they 

 approach one another in pairs, and lie in close proximity, 

 parallel one to the other. A communication is then established 

 by means of short connecting tubes between the chambers of 

 adjacent filaments, and across the channel thus formed the whole 

 of the protoplasm of one of the conjugating chambers passes into 

 the cavity of the other, and then immediately fuses with the pro- 

 toplasm it finds there. The single mass thus formed shapes 

 itself into a solid oval body, known as a "zygospore." This 

 now frees itself from the filament, secretes over its naked surface 

 a new wall of cellulose, and, when placed in the conditions 

 necessary for its development, attaches itself by one end, and 

 then, by repeated acts of cell division, grows into a many-celled 

 filament like those in which it originated. 



The formation of plasmodia, regarded as a coalescence and 

 absolute fusion into one another of separate naked masses of 

 protoplasm, is a phenomenon of great significance. It is highly 

 probable that, notwithstanding the complete loss of individuality 

 in the combining elements, such differences as may have been 

 present in these will always find itself expressed in the proper- 

 ties of the resulting plasmodia — a fact of great importance in its 

 bearing on the phenomena of inheritance. Recent researches, 

 indeed, render it almost certain that fertilisation, whether in the 

 animal or the vegetable kingdom, consists essentially in the 

 coalescence and consequent loss of individuality of the proto- 

 plasmic contents of two cells. 



In by far the greater number of plants the protoplasm of most 

 of the cells which are exposed to the sunlight undergoes a curious 

 and important differentiation, part of it becoming separated 



from the remainder in the form usually of green granules, known 

 as chlorophyll granules. The chlorophyll granules thus consist 

 of true protoplasm, their colour being due to the presence of a 

 green colouring matter, which may be extracted, leaving behind 

 the colourless protoplasmic base. 



The colouring matter of chlorophyll presents tmder the .spec- 

 troscope a very characteristic spectrum. For our knowledge of its 

 optical properties, on which time will not now permit me to dwell, 

 we are mainly indebted to the researches of your townsman, Dr. 

 Sorby, who has made these the subject of a series of elaborate 

 investigations, which have contributed largely to the advance- 

 ment of an important department of physical science. 



That the chlorophyll is a living substance, like the uncoloured 

 protoplasm of the cell, is sufiiciently obvious. When once 

 formed, the chlorophyll granule may grow by intussusception of 

 nutriment to many times its original size, and may multiply itself 

 by division. 



To the presence of chlorophyll is due one of the most striking 

 aspects of external nature — the green colour of the vegetation- 

 which clothes the surface of the earth ; and with its formation 

 is introduced a function of fundamental importance in the eco- 

 nomy of plants, for it is on the cells which contain this substance 

 that devolves the faculty of decomposing carbonic acid. On 

 this depends the assimilation of plants, a process which becomes 

 manifest externally by the exhalation of oxygen. Now it is 

 under the influence of light on the chlorophyll-containing cells 

 tliat this evolution of oxygen is brought about. The recent 

 observations of Draper and of Pfeffer have shown that in this 

 action the solar spectrum is not equally effective in all its parts ; 

 that the yellow and least refrangible rays are those which act 

 with most intensity ; that the violet and other highly refrangible 

 rays of the visible spectrum take but a very subordinate part in 

 assimilation ; and that the invisible rays which lie beyond the 

 violet are totally inoperative. 



In almost every grain of chlorophyll one or more starch granules 

 may be seen. This starch is chemically isomeric with the cellu- 

 lose cell wall, with woody fibre, and other hard jiarts" of plants, 

 and is one of the most important products of assimilation. 

 When plants whose chlorophyll contains starch are left for a .-, 



sufficient time in darkness, the starch is absorbed and completely 

 disappears ; but when they are restored to the light the starch 

 reappears in the chlorophyll of the cells. 



With this dependence of assimilation on the presence of 

 chlorophyll a new physiological division of labour is introduced 

 into the life of plants. In the higher plants, while the work of 

 assimilation is allocated to the chlorophyll-containing cells, that 

 of cell division and growth devolves on another set of cells, 

 which, lying deeper in the plant, are removed from the direcS 

 action of light, and in which chlorophyll is therefore never pro- 

 duced. In certain lower plants, in consequence of their simpli- 

 city of structure and the fact that all the cells are equally exposed 

 to the influence of light, this physiological division of labour 

 shows itself in a somewhat different fashion. Thus in some of 

 the simple green algre, such as Spirogyra and Hydrodictyon, assi- 

 milation takes place as in other cases during the day, while their 

 cell division and growth takes place chiefly, if not exclusively, at 

 night. Strasburger, in his remarkable observations on eel! 

 divisions \xi Spirogyra, was obliged to adopt an artificial device in 

 order to compel the Spirogyra to postpone the division of its cells 

 to the morning. 



Here the functions of assimilation and growth devolve on one 

 and the same cell, but while one of these functions is exercised 

 only during the day, the time for the other is the night. It 

 seems impossible for the same cell at the same time to exercise 

 both functions, and these are here accordingly divided between 

 different periods of the twenty-four hours. 



The action of chlorophyll in bringing about the decomposi- 

 tion of carbonic acid is not, as was recently believed, absolutely 

 confined to plants. In some of the lower animals, such as 

 Stentor and other infusoria, the Green Hydra, and certain green 

 planaria: and other worms, chlorophyll is differentiated in their 

 protoplasm, and probably always acts here under the influence of 

 light exactly as in plants. 



Indeed, it has been proved ^ by some recent researches of Mr. 

 Geddes, that the green planarias when placed in water and ex- 

 posed to the sunlight give out bubbles of gas which contain from 

 44 to 55 per cent, of oxygen. Mr. Geddes has further shown 

 (^hat these animals contain granules of starch in their tissues, and 



> " Sur la Fonction de la Chlorophyll dans les Flanaires vertes," Compter 

 Kendus, December, 1878. 



