414 



LECTURE XXVI. 



similar occurs, it is true, in t(ie case of certain pliants also, the' so-called annuals ; 

 but the vast majority of plants owe the continuity of their existence chiefly to the 

 circumstance that, besides organs already fully developed, new growing-points capable 

 of development are continually being originated, "by means of which the life and 

 growth of the same plant-specimen is continued from year to year, or even from 

 century to ceiitury. It is owing to this fact, also, that the idea of the ' individual ' 



i,e. that which is only capable of indivisible existence — can find no rational 



application whatever in the case of the great majority of plants ; since it is mere 

 trifling with empty terms either to distinguish with Schleiden each individual cell, 

 or with Alexander Braun, each shoot-bud emphatically as an individual. At any 

 rate no deeper insight into the nature of plants can be obtained in that way. 



Plants, therefore, in the condition in which they are usually observed (i.e. apart 

 from the microscopically minute primary embryonic state), always consist partly of 

 mature, fully-developed portions which no longer grow, and partly of immature por- 

 tions ; these latter, as in the case of winter-buds, may be dormant, but subsequently 

 develope further, or they may be portions which are developing — i. e. increasing in 

 volume and changing in form. In order to guide himself aright even in the most 

 elementary doctrines of vegetable physiology, it is absolutely necessary for the 

 student to acquire the clearest possible idea of these relations : the theory of growth, 

 especially, has no sense or meaning whatever for him who is not sufficiently familiar 

 with these matters. I shall therefore attempt to make clear by a few examples the 

 distribution of the phases of growth in space, as well as their changes in time. It 

 may be remarked at the outset that I distinguish three phases of growth, which are 

 continually passing over into one another, but which are nevertheless sharply 

 characterised. 



Organs are first met with in an embryonic condition: growing further, they 

 enter upon a second phase, that of elongation, by means of which they attain their 

 definitive volume and their definitive external configuration. Only in a third stage of 

 growth is the internal structure also of the already elongated organs completed. 

 I distinguish this last phase of growth as that of internal development, upon which 

 the condition of being fully grown — the mature state — at last follows. 



As an example serving for the illustration of these matters, let us consider Fig. 254, 

 which represents in the form of a simple diagram the phases of growth of a young, 

 erect, dicotyledonous plant. In the unripe seed of this plant, at the proper time, an 

 embryo would be found in the form represented at /. On this are observed the grow- 

 ing-point iv) of the future shoot (plumule), and that of the radicle {w) : the two pro- 

 tuberances c c are the two primary leaves (cotyledons) of the seedling in a rudimentary 

 state. The nearly black shading of v and w is intended to indicate that these parts 

 still consist entirely of embryonal tissue; while the lighter shading oi cc indicates 

 the second phase of growth, the cell -tissue already beginning to pass over into the 

 stage of elongation. This condition of development of the embryo has resulted, 

 however, from one still earlier, where the embryo had almost the form of a sphere, 

 and where no differentiation whatever, either into various organs or into various forms 

 of tissue, was as yet to be recognised : the entire embryo then consisted of homo- 

 geneous, embryonal cell-tissue, the commencing differentiation of which is already 

 indicated in /. 



