ADVENTITIOUS GROWING-POINTS. 477 



neighbourhood of the mother growing-point, or even in it. Unfortunately, in spite 

 of innumerable examinations of the apex of the root, special investigations on this 

 question are wanting : only so much is certain, that in many cases the first visible 

 rudiments of the young growing-points of roots are already to be found a few tenths 

 of a millimetre behind the apex of the parent-root. It has likewise already been 

 mentioned in the lectures on Organography that the young roots arise progressively 

 from a special simple layer of tissue at the circumference of the axial strand of the 

 mother-root, and this, corresponding to the vascular strands, in two, three, or more 

 longitudinal rows. Since the first beginnings of these tissue-differentiations are recog- 

 nisable right up to the apex of the growing-point in the mother-root, the facts given 

 do not prove that the growing-points of the lateral roots did not already exist in 

 embryo in the embryonic tissue at the growing-point of the mother-root. While they 

 then develope very slowly, the apex of the latter grows rapidly in length, and thus 

 it appears as if they had been produced far distant from the parent growing-point. 

 In the absence of exact investigations on this point (which entail very considerable 

 diflSculties moreover), we must not overlook the possibility that the first rudiments 

 of young roots may be formed at a distance from the parent growing-point ; here, 

 however, the further assumption would present itself that the layer of tissue surround- 

 ing the axial strand, and from which the young roots originate, contains traces of 

 embryonic substance derived from the growing-point. This layer of tissue would 

 then, with respect to the formation of organs, behave much as the cambium of 

 wood-plants with respect to the formation of tissue, since we may regard the cambium 

 also as a thin layer of embryonic tissue, which, however, only in rare cases produces 

 growing-points of shoots. This is saying nothing new, since even the tissue of 

 the growing-point was formerly designated Cambium. 



We have hitherto concerned ourselves exclusively with the formation of new 

 growing-points from growing-points which already exist, or, to speak more generally, 

 from embryonic tissue already present. In this way arises the ordinary normal 

 branching of shoots and roots, and the foundation for the complete normal 

 architecture of a plant is laid. It also happens, however, that new shoots and roots 

 may originate at places where the tissue has already passed over into the permanent 

 condition : in fully developed roots, in the interfoliar parts of shoot-axes, and more 

 particularly in foliage leaves, the tissue of which is already completely differentiated 

 and developed, new growing-points of roots and shoots may appear in somewhat 

 numerous cases. I distinguish these new formations exclusively with the name 

 adventitious structures — a name which formerly signified many other things also — 

 thus implying that they constitute something merely additional, something superfluous 

 for the normal structure of the plant. This is not saying that they are superfluous 

 for the whole biology and maintenance of the plants concerned : on the contrary, 

 adventitious roots are often important organs for the support of a plant, while 

 adventitious shoots serve as organs of propagation which sooner or later become 

 separated from the mother-plant as so called bulbils, and, forming adventitious roots, 

 become independent plants. 



Adventitious roots may originate, often in large numbers, at the most various 

 places between existing lateral roots from a main root, or on shoot-axes or leaves, or 

 even close beneath the growing-point of shoots, and may be indispensable even for 



