362 The Living Plant 



little doubt that it is the beginning of the formation of these which 

 gives the stimulus to the formation of sepals, petals, stamens, and 

 pistils, instead of ordinary leaves. But it is not yet clear what it is 

 which starts this formation of pollen grains and embryo-sacs, 

 though it must result in part from some outside stimulus, since 

 plants can be made to flower much sooner by making the external 

 conditions somewhat harder. The flower, once formed, secures 

 pollination or cross pollination preparatory to fertilization, as 

 described in our earlier chapters, and is followed by the fruit 

 which aids in dissemination, as we shall consider in the chapter 

 that follows. But with the flower, indeed, we are back to the 

 fertilized egg-cell with which we began, and thus is the cycle 

 completed. 



A matter of very much interest in connection with the growth 

 of plants from the seedling to the adult concerns the changes in 

 their tissues. The tissue of the young embryo is all capable of cell 

 division (is meristematic, in anatomical language), but as the 

 embryo germinates, only the tip of the root and the first bud, to- 

 gether with a thin hollow cylinder of cambium connecting them, 

 remain so, while all the remainder of the cells grow large, assume 

 special functions, and lose their power of division. The new 

 growing points as they originate, whether on stems or on roots, 

 establish connections with this cambium cylinder so that to- 

 gether they form one continuous system, in which all of the grow- 

 ing points are connected with one another by hollow cylinders of 

 cambium, and conversely, the cambium cylinder branches into 

 numerous tapering tubes terminating in the growing points, as 

 our diagrammatic figure illustrates (figure 139). Meantime the 

 cambium grows steadily outward, as the growing points grow 

 steadily onward, each forming permanent tissues behind them. 

 This separation of growth and permanent tissues makes it possible 

 for a plant to go on growing without limit, and were it not for the 

 restrictions imposed by external physical conditions, there is no 

 reason why trees should not be immortal. In this possession of 



