52 Growth 



more of them parallel to the primordium axis, in the direction of growth 

 in length, than at right angles to it, in the direction of growth in width. 

 In isodiametric ovaries, however, spindle angles were almost equally 

 distributed between and 90° to the axis (Fig. 3-17). 



The question arises as to just what determines the plane of cell division 

 in cases like this. It cannot be simply the orientation of the mitotic spin- 

 dle, for this can be shown to change during mitosis. Thus in the fruit of 

 Trichosanthes, which is very long and narrow, practically all the divisions 

 are transverse to the long axis and their orientation can thus be predicted. 

 The spindles in metaphase, however, are by no means all parallel to the 

 axis but vary considerably. In anaphase the variation is much less, and 

 in telophase the cell plates are almost all transverse ( Fig. 3-18 ) . Evidently 

 the spindle rolls about somewhat during mitosis (as it has been seen to 

 do in living material of other forms ) but finally settles into position. What 

 this position will be seems to be determined by the cytoplasmic body 

 since, in vacuolate cells, the phragmosome is formed at prophase in the 

 position of the final cell wall (p. 25). 



Something certainly controls not only the plane of cell division but the 

 distribution of divisions and the amount and character of cell expansion. 

 Whatever this may prove to be, it is concerned with the origin of organic 

 form. If one looks at a section through a young and growing plant struc- 

 ture, such as an ovary primordium, he sees a mass of cells of various 

 shapes and dividing in many planes. Here chaos seems to reign. When he 

 observes how such a structure develops, however, and finds that it is 

 growing in a very precise fashion, each dimension in step with all the 

 others, he comes to realize that this is not the seat of chaos but of an 

 organizing control so orderly that a specific organic form is produced. 

 This realization is one of the most revealing experiences a biologist can 

 have and poses for him the major problem that his science has to face. 



CELL SHAPE 



One of the simplest manifestations of organic form is in the shape of 

 individual cells. This obviously involves plane of cell division, cell size, 

 polarity, microstructure of the wall, genetic constitution, and other fac- 

 tors. 



Since the cell, at least at first, is a fluid system, its natural shape, other 

 things being equal, is that of a sphere, for this has the least surface in 

 proportion to its volume. Most cells, however, are parts of tissues and 

 thus are closely packed against neighboring cells on all sides. This re- 

 sults in a modification of the basic spherical shape to that of a polyhedron 

 with flattened sides, each representing a plane of contact with an adja- 

 cent cell. How many faces should such a cell have, and what sorts of 



