434 



LECTURE XXV IT. 



FIG. 268.— The 



spherical cell divided 



spheres by a division wall. Of these, the one has become elongated in a vertical, 

 and the other in a horizontal direction ; each of the two halves, however, is 

 divided by a cross-wall produced at right angles to the long axis of the secondary 

 cell, and thus have arisen two pairs of cells in a crossed position. The mother-cell 

 F was probably more completely spherical than the previous one, and, accordingly 



also, the form of division is different. The 

 mother-cell has here become divided in a 

 tetrahedral manner, as we are accustomed 

 to say — i.e. six division-walls have been 

 produced simultaneously, in such a way that 

 the spherical mother-cell has become cut 

 up into four daughter-cells, each of which 

 would resemble a tetrahedon if the outer 

 wall were plane instead of arched out- 

 wards. The position of these division 

 walls is illustrated in Fig. 268, and it may 

 be recognised at once, that in this case 

 the six walls simultaneously produced do not cut one another at right angles. It 

 may be shown, however, that this case can be brought with that of the ordinary 

 rectangular cutting of the succeeding division-walls under a common general 

 expression. 



In all these various cases of the division of a mother-cell, the rule, also common 

 elsewhere, makes itself evident, that the daughter-cells which arise simultaneously 

 are equal in volume to one another — i.e. the division consists in a halving of the 

 mass of the mother-cell. This rule also of course suffers various exceptions in 



particular cases : otherwise, however, it is 

 so comprehensive that it may always be 

 laid down as the ordinary case. 



As the general rules of successive cell- 

 division in growing organs, therefore, we 

 can already state (i) that the daughter- 

 cells are usually equal to one another in 

 volume, and (2) that the new cell-walls are 

 situated at right angles to those already 

 present. 



Simple though these two rules may ap- 

 pear, it is nevertheless difficult to prove 

 their validity in many instances, according 

 to the form of the mother-cell in each case. 

 For example, when the dividing-cell has 

 the form illustrated in Fig. 269 — i.e. the shape of a tetrahedron with curved 

 surfaces — and if the successive divisions in it halve the volume each time, and in 

 doing so stand at right angles on the preceding walls, a structure is obtained like 

 that shown in the figure, from which the existence of the relations above mentioned 

 can only be determined by careful geometrical considerations. 



That the mode of cell-division depends only upon the increase in volume and 



Fig. 269.— Diagram of a tetrahedral apical cell abcd& met 

 with in Equisetum and elsewhere, seen from above, de, fs, 

 h k, the walls of three successive divisions ; i the corner where 

 the three walls cut one another like the sides of a cube. 



