CELLS AND TISSUES 13 



the genetit or formative tissue, in contrast to those which retain 

 this quality but seldom express it except under unusual circum- 

 stances (potential meristems), or which have no capacity for 

 further growth and differentiation (non-meristems). The cells in 

 meristematic regions are characterized by their approximate 

 isodiametric form, except for some of the cambial initials of 

 lateral meristems; the presence of a functional protoplast which is 

 capable of active nuclear and cellular division; and a primary wall 

 that may grow and enlarge or undergo reversible changes in thick- 

 ness. The wall is composed largely of cellulose and possesses 

 plasmodesma in varying degrees of frequency and aggregation. 



Factors of Differentiation. — The tissues which form the 

 complex structure of the plant are derived from the meristems, 

 but the causes which underlie differentiation and the factors which 

 control the processes concerned in it are still incompletely under- 

 stood. Geneticists have provided a theoretical unit of heredity 

 in the gene^ and the transmission of heritable characters from one 

 cell generation to another through nuclear division has been shown 

 to be related to this remarkably regular and normally precise proc- 

 ess. This aids in the understanding of many of the phenomena of 

 vegetative reproduction, and variation in nuclear behavior may 

 explain some of the problems of mutation and variation in the 

 species; but it is yet to be determined why two adjacent cells of 

 common origin, and presumably with the same hereditary pattern, 

 should differentiate into elements that appear totally unlike each 

 other in structure and function. The factors which are responsible 

 for this differentiation may lie partly within the cell; but undoubt- 

 edly the mechanical, physical, and chemical stimuli of the sur- 

 rounding environment play an important part in the process. 



In this connection, continued work with tissues and organs in 

 culture such as that initiated by Robbins (36), White (44), LaRue 

 (17), and Bonner (8) should increase our knowledge of the factors 

 involved in differentiation. Recent work by Went and Thimann 

 (43), Kraus and his coworkers (x6), Boysen-Jensen (10), and others 

 on phytohormones and their relation to growth and development 

 offers an additional avenue of information regarding this subject. 



It seems probable that the positional relationships of cells 

 may play some part in the character of their differentiation. 

 Seeliger (39) points out in his study of sugar beets that "in the 

 development of supernumerary tissues it is not the morphological 



