284 



ME TAMORPHOSIS 



briefly the ' continuity of embryonic substance ' applies to the root as well as to 

 the stem. [As to this embryonic substance, compare Noll, 1903.] Every 

 growing point is a portion of an older one ; all arise in the long run from the 

 growing points of the embryo, and these in turn come from the egg-cell. The 

 egg-cell, however, is itself formed from the growing point of the parent plant. 



We must not forget to note that in addition to this normal development 

 of new growing points there is also another method. One example of this 

 phenomenon may be referred to. The much-cultivated Begonias are multiplied 

 by laying isolated leaves on wet sand. New buds then develop from certain 

 epidermal cells (Hansen, 1881), which in turn speedily form roots and become 

 independent. As we have seen, the leaf generally ceases to grow at a very 

 early stage and the epidermal cell of a fully-developed leaf is a type of a per- 

 manent tissue element. So long as the conditions are normal, such a cell will 

 not show any visible signs of capacity for growth. Since, notwithstanding, 

 this cell becomes, under certain conditions, a growing point it proves that the 

 capacity for becoming such was latent only, and became actual on the applica- 

 tion of a stimulus. Such cases of 

 adventitious origin of growing points are 

 remarkably widespread. They teach 

 us that the difference between fully- 

 developed and embryonic cells is a 

 quantitative and not a qualitative one, 

 and the correctness of this view will be- 

 come more evident when we study the 

 subject of cambium (Lecture XXIII). 

 Turning back once more to simple 

 plants and plant parts, we see that in 

 many cases the most vigorous growth 

 occurs at the growing point, and that 

 just behind it fully-developed regions 

 are met with. We have already (p. 261) 

 studied such cases in considering the 

 apical growth of fungus cells and root- 

 hairs, but similar phenomena are to 

 be seen in more complicated growing 

 points which form lateral branches, 

 e. g. in Caulerpa, where also growth 

 rapidly ceases behind the apex (compare Reinke, 1899, p. 61). The same 

 appearance is shown by multicellular Algae. In Stypocaulon, for example, 

 the whole growth takes place in the apical cell, and cell division follows in 

 completely developed segments of the apical cell. Doubtless, detailed research 

 would establish the same results in many other plants. Nor is it remarkable 

 that it should be so, for there are plenty of plants in which nothing else is 

 possible than the localization of growth in embryonic cells since they consist of 

 such cells only. 



In contrast to this case stands another type represented especially by 

 the higher plants. Growth at the apex of the growing point is very restricted, 

 and the plants attain their proper length only by extension of cells which are 

 situated at a greater or less distance from the apex of the growing point. Thus 

 we must follow Sachs in distinguishing a primary growth period, when the 

 members are laid down, a secondary when they are elongated, and usually also 

 a tertiary when their internal differentiation is completed. [Berthold (1904) 

 attempts to distinguish six growth regions : (i) the initial region (the apex of 

 the growing point) ; (2) the region of morphological subdivision ; (3) the region 

 of anatomical differentiation ; (4) the region of elongation ; (5) the region of 



j/p^ 



U 



Fig. 81. Portion of a longitudinal section througli 

 the main root of Reseda, g^ \essels ; />, pericycle ; e, 

 endodermis ; r, cortex. After V.^N Tiegheai (Ann. 

 sc. nat. 1898, ser. 7, vol. 8.) 



