CELL. 



[ 145 ] 



CELL. 



position, a partition i,« formed dividing the 

 pavent-coll into two or more parts. The 

 form of tlie secondary cells depends of course 

 on that of the parent-cell at the time of 

 division. In the case of cellulose tissues, 

 such as those in the pvnclum irf/efafionis of 

 the buds of the h igher plants, in camhitrm ,k c, 

 the division is ordinarily into two parts, 

 which respectively grow until equal in size 

 to the pirent ; and either both or only one 

 of these divides again in the same way, and 

 so on, luitil the whole structure is com- 

 pleted. It is evident that the external 

 forms of all cellular structures must depend 

 greatly upon the laws of division of the 

 cells of plants. For example, supposing we 

 start from a single square cell, when this 

 divides into two halves, and these grow to 

 equal the parent-cell, we have an oblong 

 figure ; if the half-cells divide again in the 

 same direction, we shall in time get a long 

 filament ; and if both new cells divide again 

 each time, the filament will grow much 

 longer in a given time than if only the end- 

 cell continually divided, leaving one new cell 

 behind it at each division. If the pair of 

 cells produced by the first halving divide at 

 right angles to "the first division, a square 

 gi-oup of four cells results ; and if this law 

 continues to act, a flat plate of cellular tissue 

 will result. Further, if the cells also divide 

 by horizontal partitions ( in the third direc- 

 tion of space), the mass of cells will gradually 

 acquire thickness or height as well as length 

 and breadth. Lastly, if the cells of parti- 

 cular regions cease to divide sooner than 

 others, irregular or complex but definite 

 structures w411 be produced — as those parts 

 where the cell-division goes on will emerge 

 from the general mass, in the Cellular plants 

 as lobes, and in the higher plants as conical 

 bodies which are gradually developed under 

 similar laws into the organs. The diver- 

 sities of internal organization depend also 

 to some extent on the same laws, but less on 

 these than on the laws regulating the forms 

 which the cells acquire when fuU-grown. 



Cell-division may be observed most easily 

 in the lower Cellular plants, or in the simpler 

 structures (such as hairs) of the higher plants 

 (PI. 47. figs. 8, 9). The Confervfe afford 

 exceedingly favourable opportunities, as do 

 also the filamentous or thalloid structures 

 of germinating Mosses, Ferns, microscopic 

 Fungi, t&c. The behaviour of the parent- 

 cell before division exhibits some diversities. 

 If a simple filament is increasing by cell- 

 division, the cylindrical parent-cells merely 



elongate a little before dividing transversely. 

 If the filament is to branch, the wall of the 

 parent-cell bulges out gradually at the point 

 where the branch is to appear ; the bulging 

 soon becomes a pouch, and this pouch is 

 soon shut off" by the formation of a partition 

 at its base. Bead-hko rows of cells Ukewise 

 divide by budding in this way, as may be 

 observecl, for instance, in the Yeast-plant : 

 the new cell first appears as a little 'bubble' 

 on the side of the parent, with its cavity 

 continuous; and after it has acquired a 

 certain size, its protoplasm detaches itself 

 from that of the parent, and a partition is 

 formed at the point whence the second cell 

 emerged (PL 20. fig. 23). 



Another point which must be noticed 

 here, is the question whether the parent 

 protoplasm divides instantaneously, at a given 

 epoch, into the new utiicles, or whether it 

 parts gradually, by a sort of constriction ad- 

 vancing from the surface towards the centre, 

 roughly comparable to what occurs when a 

 ligature is slowly drawn tight round an 

 elastic tube, or w^hen a bar of soap is cut in 

 two by passing a string round it and gra- 

 dually drawing the loop tight. It seems 

 probable that the segmentation of the pro- 

 toplasm is always gradual ; and it is certain 

 that it is so in many cases. Its gradual 

 constriction has been observed in those 

 Confervae where the protoplasm forms a 

 hollow sac, lining the whole internal sur- 

 face of the parent-cell ; it may be traced in 

 the larger Confervae, in Spirogyra, &c., by 

 keeping the plants growiug in water under 

 the microscope. It appears that the division 

 is generally completed during the earlier 

 hours of the morning. 



2. Cell-division icith liberation of the new 

 cells. — The first step in this process is ana- 

 logous to what takes place at the outset in 

 the preceding set of cases; but we find 

 much more important modifications here. 

 This is the mode of development of spores 

 of the Ascomycetous Fungi, of the spores 

 and tetraspores of the Algae, the spores 

 of Lichens, the spores of all the higher 

 Cryptogamia, the active gonidia or zoospores 

 of "the Algte, the parent-cells of the sper- 

 matozcids or active spiral filaments of the 

 higher Cryptogamia, and of the pollen-grains 

 of the Flowering plants. 



The general character is : Division of the 

 whole protoplasm into segments, which 

 either acquire a cellulose coat within the 

 parent-cell before they are set free by its 

 solution or bursting, or escape from the 



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