30 Growth 



burger ( 1893 ) believed that the nucleus has a certain "working sphere" 

 and that this limits the size to which a particular cell will grow. 



Studies of comparative cell size have often been made but chiefly on 

 mature cells. This problem, however, is one that must be attacked devel- 

 opmentally. Two processes are involved in it: the rate and amount of 

 growth or increase in size in a given region and the rate and duration of 

 cell division there. Cell size is the result of the relationship between these. 

 The faster the cells divide, in proportion to the total amount of growth, 

 the smaller will they be, and vice versa. 



At the end of a cell division each daughter cell is about half the volume 

 of its mother cell. At this time it begins to enlarge, and if it is in a meri- 

 stematic region it will soon divide again. Where division rate is relatively 

 rapid, the cells may divide before they have time to enlarge to the size of 

 their mother cells, and cell volume decreases, as in some early embryos. 

 Sometimes the new cells do not expand at all and a process of cleavage 

 takes place, much as in the first stages of many animal embryos, where 

 the egg is cut up into a mass of smaller and smaller cells. In plants such 

 cleavage may be seen in endosperm formation; in the development of the 

 female gametophyte within a megaspore; in the renewal of meristematic 

 activity in large, mature cells during regeneration; and elsewhere. Where 

 cell enlargement is relatively rapid, the cells will become larger than their 

 mother cells before they divide again and cell size in the meristematic 

 region will increase. Thus in growing gourd ovaries, where the tissues are 

 still meristematic and the cells all start from a very small size, they gradu- 

 ally enlarge, though not as fast as the ovary itself. In most roots, dividing 

 cells increase in size with increasing distance from the root tip. 



In their detailed analysis of the growth of the oat coleoptile, Avery and 

 Burkholder ( 1936 ) found that in the outer epidermis cell division ceased 

 after the organ was first initiated, so that during all later growth cells here 

 elongated greatly, sometimes becoming 150 times as long as at the begin- 

 ning. The inner tissues, however, grew in part by cell multiplication until 

 the coleoptile was 10 to 20 mm. long. There were progressively fewer 

 divisions from the subepidermis inward. Thus at maturity the longest cells 

 were in the outer epidermis, the shortest in the layer next below it, and 

 the cells then increased in length toward the inner layers (Fig. 3-6). 



In some meristems, especially the vascular cambium, the size of divid- 

 ing initials may undergo permanent increase. Sanio ( 1873 ) and others 

 since his time have found that xylem cells in trees have different lengths 

 at different distances from the center of the trunk or from the ground and 

 that these differences are mainly established in the fusiform initials in the 

 cambium from which the mature cells develop. 



Most increase in cell size, however, comes after the final division. In 

 plants, as contrasted with animals, this increase may be very great. Usually 



