12 Growth 



In most multicellular organisms growth is accomplished chiefly by cell 

 multiplication, and to some observers this process seems to be an essen- 

 tial part of growth. In the cleavage of many animal eggs and in similar 

 processes in plants, however, there is a great increase in cell number 

 but none in the actual material which constitutes the "growing" structure, 

 and whether such cellular increase should be regarded as growth is a 

 question. In the case of the female gametophyte in the megaspore of 

 Selaginella, and especially of the young embryo which develops there, 

 growth of an organized structure by cell multiplication certainly occurs, 

 but at the expense of material stored in the spore. Where an entire mass 

 is cut up into cells, as in the development of a male gametophyte in a 

 microspore, one may doubt as to whether this should be called growth 

 at all. What definition of growth one adopts depends on the particular 

 problem with which he is concerned. 



For the study of morphogenesis, the most important aspect of growth 

 is the permanent increase in volume of an organ or organism, regardless 

 of how it is accomplished, and this is the sense in which the term will 

 here be used. The ultimate problem-the self-multiplication of living ma- 

 terial-is one primarily for the student of physiology and reproduction, 

 but it is the gross and geometrical result of such growth with which 

 morphogenesis is chiefly concerned. 



Growth in Plants. In most plants, the process of growth is different in 

 one important respect from that in animals. The typical mature plant 

 cell is surrounded by a relatively stout cellulose wall which under ordi- 

 nary conditions prevents any further cell division or growth save in ex- 

 ceptional circumstances. The cells are rather firmly cemented together 

 and thus unable to move about or migrate. Plant tissues are therefore in- 

 capable of growth and renewal except through the activity of thin-walled, 

 relatively undifferentiated embryonic regions, or meristems, where occur 

 the divisions that produce new cells and the changes by which these at- 

 tain their final size. These meristems are rather sharply localized. In plant 

 axes where growth is continuous and often indeterminate, growth in 

 length is controlled chiefly by the activity of meristems at the tip of each 

 root or stem. The older portion of the axis, having once attained maturity, 

 does not make further growth in length. A tree increases in height only 

 at the tips of its twigs and not elsewhere. In the stems of some mono- 

 cotyledons, however, growth of the stem in length may continue for a time 

 by the activity of intercalary meristematic regions at the base of each 

 internode. Perennially growing roots and stems increase in thickness 

 through the activity of a lateral meristem, or cambium, situated between 

 xylem and phloem, by which the growth of both these tissues is accom- 

 plished. There are other sharply localized meristematic regions, such as 

 the phellogen, or cork cambium. 



