70 



GENERAL BOTANY 



like leaves, fruits, seeds, and tubers, usually grow more like animals, 

 by a uniform increase in the size of cells throughout the entire 

 structure. Cylindrical organs, on the contrary, like roots and stems, 

 which have periods of enlargement often extending over many 

 years, have definitely located growing masses, or layers, of cells, called 

 meristems and cambiums. These meristem and cambium cells renew 

 their cell divisions each season, in the case of perennial organs, 



and contribute new 

 cells, which then pass 

 through the second 

 and third phases of 

 growth to form new 

 tissues and organs. 

 The apical meristems 

 which terminate the 

 roots, the stems, and 

 the branches have al- 

 ready been studied. 

 The cambiums are 

 cylindrical layers of 

 meristem cells lying 

 between the wood and 

 the bark in steins and 

 roots and between the 

 outer cork and inner 

 bark. These cam- 

 biums enable roots 



FIG. 39. Diagrams illustrating the method of elon- 

 gation of roots and herbaceous stems 



a, 6, elongation of the root tip illustrated as in Fig. 34; 



c, (/, similar method of representing the elongation of a 



herbaceous stem like the bean (Fig. 36). Consult the 



text for further discussion 



and stems to grow 

 in diameter and also 



to form new annual tissue layers for protection and for the con- 

 duction and storage of foods, water, and soil salts. The apical 

 meristems also enable plants to send out new leaves and roots each 

 season for the absorption of raw food elements and the compounding 

 of these *raw materials into organic foods. 



The above general statements relative to the growth of stems and 

 roots by means of apical meristems and cambium layers will be more 

 readily understood by reference to the following diagrammatic figures: 

 Fig. 39, a and b, is a repetition of Fig. 34, illustrating the method 

 of growth of roots in length by means of an apical meristem. Fig. 39, 

 c and d, illustrates in a similar manner the growth of a herbaceous 



