Ways in Which Plants Increase in Size 341 



of a definite number of the bodies called chromosomes; and these 

 become arranged in a plate across the cell. Meanwhile the bound- 

 ary of the nucleus has vanished, and a spindle-shaped framework 

 of very fine fibers has formed at right angles to the chromosome 

 mass. Then each chromosome splits lengthwise into two, and the 

 spindle draws these halves apart towards its two ends, where 

 they become surrounded anew by a nuclear boundary. Thus is 

 the chromosome matter divided evenly between the two new 

 nuclei. The chromosomes then lose their distinctness and grad- 

 ually merge away to the threads, and finally to a granulation 

 similar to that of the original nucleus. Meantime the spindle 

 fades away and a new wall forms across the cell between the new 

 nuclei. Each of the new cells then grows to the original size and 

 is ready for another division. 



The object of this elaborate process is without doubt the equal 

 division of the chromosomes. These, it will be remembered, are 

 derived equally from the two parents of the plant, half of them 

 from one parent and half from the other; and although they ab- 

 sorb nourishment and grow and divide, they never lose their 

 identity. The equal division of the chromosomes in every division 

 of the cells, therefore, carries some of the substance derived from 

 each parent to every cell of the adult plant, thus explaining how 

 it is that any part of a plant can resemble either one of its parents. 



Cell division underlies all development of new parts, for every 

 structure — leaf, stem, root, or other — begins with the formation 

 of just so many cells at just such places as will produce, when they 

 swell to full size, the characteristic size and shape of the fully-adult 

 organ. But at first these cells are all small, and densely packed 

 with protoplasm and food substance. Such is the condition in a 

 bud or an embryo, as our figures illustrate (figures 137, 139, C). 

 One must not, however, lay too much stress upon the cell divisions 

 in particular, for they are without doubt a result, rather than a 

 cause, of the outgrowth of new parts. It is in reahty the living 

 protoplasm which pushes out into new structures; the cell divi- 



