220 THE CELL 



of the dividing surfaces in bipartition. For, having once learnt 

 the causes which determine the position of the spindle axes, we 

 can know beforehand how the division plates must lie, in order to 

 intersect the spindle axes at right angles. 



As a general rule, unless the mother-cell is exceptionally long 

 in any one direction, it happens that in each division that axis 

 of the daughter-cell, which lies in the same direction as the 

 chief axis of the mother-cell did, has become the shortest. Hence 

 the axis of the second division spindle would never in such a case 

 place itself in the direction of the preceding division spindle, but 

 rather at right angles to it, according to the form of the proto- 

 plasmic body. In consequence, the second division plane must 

 intersect the first at right angles. 



Generally, the consecutive division surfaces of a mother-cell 

 (which becomes split up into 2, 4, 8, and more daughter-cells by 

 successive bipartitions) lie in the three directions of space, and so 

 are more or less perpendicular to each other. 



This is often very plainly to be seen in plant tissues, because 

 here firm cell-walls, corresponding to the division planes of the 

 cells, rapidly develop, and thus, so to speak, fix the places to a 

 certain degree permanently. But in animal cells, which in the 

 absence of a firm membrane frequently change their form during 

 the processes of division, this is not the case ; in addition the 

 position of the cells to one another may change. " Fractures and 

 displacements " of the original portions into which the mother- 

 cell splits up occur, examples of which are afforded us by the 

 study of the furrowing of any egg cell. This is entered into more 

 fully on p. 224. 



In botany, these three directions of space are designated as 

 tangential or periclinal, transverse or anticlinal, and radial (Figs. 

 Ill, 112). Periclinal or tangential walls are parallel to the 

 surface of the stem. Anticlinal or transverse walls intersect the 

 periclinal walls, and at the same time the axis of growth of the 

 stem at right angles. Finally radial walls, whilst being also at 

 right angles to the periclinal ones, lie in the same plane as the 

 axis of growth of the stem. 



In order to render this clear by an example, we will select a 

 somewhat difficult object, namely, the growing-point of a shoot. 

 Sachs demonstrates the truth of his law with reference to this 

 object in the following sentences which are taken from his lectures 

 on plant physiology (II. 33) : 



