848 



A TEXTBOOK OF THEORETICAL BOTANY 



have an extensive determining influence over all those features of the mature 

 organs which, taken together, mark every species with its distinctive character. 



There are two main categories of tissue in a plant, the formative and 

 the permanent. They are complementary in function, for while the forma- 

 tive produce the permanent tissues, the latter nourish the former. Functionally 

 the formative tissues are for growth and diff^erentiation and the permanent 

 tissues for assimilation and distribution. The meristem is therefore not 

 simply one among many types of tissue in the plant, it is itself equivalent to 

 all the types of permanent tissue together. 



The name meristem means the divisible tissue, and its outstanding 







Fig. 838. — Abutilon megapotamicurn. Dicotyledon. Transverse 

 section through the apical bud showing in the centre the 

 growing point of the stem surrounded by successive leaf 

 rudiments. 



character is that of rapid and repeated cell division. The majority of meri- 

 stematic cells only keep this character temporarily and soon pass into one 

 or other of the paths of differentiation which lead towards the permanent 

 tissues, yet, paradoxically, the meristem as such is permanent. There is in 

 every meristem a terminal group of cells which never differentiate, but, like 

 queen bees, produce the cells which do. This is the initial meristem, the 

 source of all the rest (Figs. 838 and 839). The respective rates of growth 

 and of differentiation are autonomously determined in the meristem itself 

 and are so balanced that the meristem neither increases nor decreases in 

 amount over long periods. 



The initial meristem is only a part of the formative tissues at the apex. 

 Between it and the permanent tissues there is an intermediate region of 



