LECTURE XXVIII. 



FORMATION OF ORGANS AT THE GROWING-POINT: 

 BRANCHING. 



The growing-points already considered to such an extent in the two preceding 

 lectures, scarcely ever come under the notice of those not trained in botanical science, 

 or at any rate they are passed over,' not only on account of their minuteness, 

 but also because, as a rule, they are hidden from view; those of most roots because 

 they are in the earth or other substratum, those of leaf-forming shoots because they 

 are completely enveloped by the young leaves. Nevertheless, it will already be clear to 

 the reader (and it is one of the most important facts of the whole of the physiology 

 of plants) that all formative processes are initiated at the growing-points — that all the 

 organs of a plant arise from these tiny masses of embryonic tissue which we have to 

 seek in the form of growing-points at the apices of the roots, or at the ends of shoots 

 inside the buds. 



Here again, also, the profound diiference between root and shoot comes out 

 sharply. Roots produce only organs of like kind, namely, new roots, by means of 

 which the branching of the root-system is brought about ; whereas the activity of the 

 growing-points of shoots is entirely different and much more various : they produce 

 not only growing-points of new shoots, and thus of course subserve branching, but 

 from them alone also all leaves arise, and together with or on these the true 

 reproductive organs — sporangia, oogonia, and antheridia, since all reproductive organs 

 may be referred to these three forms. Thus, when in botanical investigations it is 

 important to examine organs of any kind whatever in their first stages of develop- 

 ment — in their primitive mutual relations — it is always the first object of the observer 

 to investigate the growing-points, and the formation of organs taking place on them. 

 Owing to the extreme minuteness of these objects, this has always to be done with 

 the aid of the microscope. 



Here again we shall only concern ourselves with the commonest phenomena, or 

 those which offer features of special interest, and according to the plan of exposition 

 which I have adhered to so far, I again confine myself in the first place to the 

 typical and complete forms. We may first regard the growing-points of leaf-forming 

 shoots as found in all Vascular plants, most Muscinese, and even in many Algae : 

 the abnormal forms then present no further difficulties. Before entering more closely 

 upon our proper theme, however, it may be advantageous to make a few preliminary 

 remarks as to the form of the growing-point, and its position on the shoot. 



In leaf-shoots which are growing vigorously in length, the growing-point has 



