12. 



THE STRUCTURE OF ECONOMIC PLANTS 



anatomical or physiological. For example, where the classifica- 

 tion is based upon function, conducting tissues have been called the 

 mestome and protective tissues the stereome. The mestome has been 

 further divided into the hadrome, conducting water and substances 

 in solution; and the leptome, comprised of food-conducting tissues. 

 Since these terms are not the morphological equivalents of such 

 designations as sclerenchyma, xylem, and phloem, their use in 



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Fig. 4. A quadrant of the root tip of corn illustrating the progressive differentiation of 

 meristematic cells to form vessel segments, parenchymatous cells, epidermis, and root cap. 



anatomical descriptions may result in confusion, and they are not 

 used in the succeeding chapters. 



Meristems. — All the potentialities for development which are 

 ultimately expressed in the mature seed plant reside in the fertil- 

 ized egg or Xjgote; and, in the embryo derived from it, these are 

 transmitted by the actively dividing nuclei of the cells which form 

 the terminal, intercalary , and lateral meristems of the axis. (Fig- 40 

 In developmental anatomy, tissues may be further classified as 

 being actively meristematic, potentially meristematic, or non- 

 meristematic. Active meristems are cell aggregates constituting 



