BRANCHES AND ROOTS. 81 



and not (as in the case of the false roots or rhizoids of the pro- 

 thallium, shortly to be described) by elongation of superficial 

 cells of the epidermis. True roots, of which those of Pteris are 

 good examples, arise always as well from the fundamental and 

 fibre-vascular regions, and include all the systems found in the 

 stem itself. Hence cross-sections of Pteris roots differ but slightly 

 from those of the stem or the branches, and the root in general 

 is clearly a member of the plant body. As in all true roots, the 

 free end is covered by a special boring tip called the root-cap, but 

 this is apt to be lost in removing the specimen from the earth. 



GKOWTH OF THE RHIZOME. 



The Embryonic Tissue or Meristem. The starting-point in the 

 development of the underground parts will be fully described 

 hereafter (p. 98). But briefly stated it is as follows : By con- 

 tinued division of the germ-cell (compare p. 51) a mass of similar 

 cells is formed, part of which is destined to develop into the rhi- 

 zome. By gradual differentiation (p. 15) of the cells, this portion 

 is converted into the mature rhizome, which however remains 

 nearly nndifferentiated at the tip. At this point the epidermis 

 may be distinguished, but it remains very delicate, and the under- 

 lying cells continue to grow and multiply, thus producing con- 

 tinued elongation of the mass. In this way the first apical bud 

 is formed. Lateral buds are then given off right and left to con- 

 stitute the embryos of leaves, branches or roots, which, still re- 

 taining their soft and delicate tips, are capable of further growth. 



Behind these " growing points " the epidermis and other 

 tissues grow more and more slowly, and soon reach their maxi- 

 mum size, whereupon rapid growth ceases. The power of growth 

 is henceforward mainly confined to the apical buds, and the grow- 

 ing tissue of which they are composed is known as embryonic 

 tissue or meristem. 



The Apical Cell. Close examination reveals the fact that each 

 apical bud contains a remarkable cell which is especially concerned 

 in the function of growth, viz., the apical cell, which lies in a hol- 

 low at the apex of the bud. In the apical buds of the rhizome or 

 branches this cell has somewhat the form of a wedge-shaped 

 pyramid. It continually increases in size, but as it grows, repeat- 

 edly divides so as to cut off cells behind it alternately on its right 

 6 



