12 



INTRODUCTION TO CYTOLOGY 



cells in the cambium. As a rule, meristematic cells contain no conspic- 

 uous ergastic inclusions aside from vacuoles and are separated by very 

 delicate walls with no intercellular spaces. As growth proceeds in such 

 tissues, every nuclear division is accompanied by a division of the cyto- 

 plasm; hence growth results in the multiplication of the uninucleate cells. 

 In regions farther from the point of greatest meristematic activity the 

 cells gradually become visibly diversified in structure in connection with 



their increasing specialization in func- 

 tion. Throughout the active life of the 

 plant the homogeneous meristematic cell 

 mass thus continues to grow distally 

 and differentiate proximally into tissues 

 with very diverse histological characters 

 (see Eames and MacDaniels, 1925). 



Fig. 7. — Longitudinal and transverse sections of apical meristem in root of Pteris, 

 showing triangular pyramidal apical cell. The segments cut from the distal face of the 

 cell go to form the root cap, while those from the three lateral faces develop into the tissues 

 of the root proper. In seed plants the meristematic activity does not center in a single 

 distinguishable cell. {After Hof, 1898.) 



Most of the visible characters which ordinarily serve to distinguish 

 the various kinds of differentiated cells of the vascular plant are found 

 in the cell wall rather than in the protoplast itself. Thus, besides meri- 

 stematic and slightly modified parenchymatous cells, there are many 

 other types, such as tracheids, vessels, wood fibers, phloem fibers, and 

 sieve tubes (Fig. 8), all of which are characterized by the peculiar ways in 

 which their walls become modified through secondary and tertiary thick- 

 enings and by the form and arrangement assumed by the pits (see p. 175). 

 The protoplasts may finally disappear completely from wood cells, 

 leaving a tissue or framework composed of lifeless cell walls. All func- 

 tional differences are accompanied by chemical or physical differences of 

 some sort in the protoplasm, but it is mainly in the non-protoplasmic 



