10 STRUCTURAL AND PHYSIOLOGICAL BOTANY. 



66. An embryo is a young plant, produced by the agency of 

 sexes, and developed within a seed. 



67. A leaf-bud is a young plant, produced without the agency 

 of sexes, enclosed within rudimentary leaves called scales, and 

 developed on a stem. 



68. An embryo propagates the species. 



69. Leaf-buds propagate the individual. 



70. When the vital action of an embryo or bud is excited, 

 the tissue developes in three directions, upwards, downwards, 

 and horizontally. 



71. That part which developes downwards is called the 

 descending axis or root ; that upwards, the ascending axis or 

 stem ; that horizontally, the medullary system ; and the part 

 from which these two axes start is called the crown or collar. 



72. This elongation in three directions takes place simul- 

 taneously ; hence it follows that all plants must necessarily 

 have an ascending and descending axis, or a stem and root, 

 and a medullary system. 



73. The only apparent exceptions to this are the lower 

 tribes of plants, in which the developement seems to be either 

 spherical, filamentous, or horizontal. 



III. ROOT. 



74. The root is formed by the descending and dividing 

 fibres of the stem. 



75. Anatomically it differs from the stem in the absence of 

 normal buds, and of stomates (49), and in Exogens of pith. 



76. Although the root has no distinct pith in Exogens, yet 

 it possesses a distinct medullary system. 



77. The functions of the root are to fix plants in the earth, 

 and to absorb nutriment from it. As it has to force its way 

 through substances which offer resistance to its passage, it 

 lengthens exclusively by successive additions to the points 

 of its divisions. 



78. This absorption takes place almost exclusively by the 

 extremities called spongelets, or spongioles, which consist of a 

 lax coating of cellular tissue lying upon a concentric layer of 

 woody tissue, in the midst of which is often placed a bundle 

 of ducts (31). Spongioles are not, however, a distinct organ, 

 but are merely the young extremities of roots. 



