ACROPETAL VEGETATIVE REPETITION. 



467 



repetition from one and the same growing-point. This is most distinctly seen, 

 however, in the formation of leaves. 



When organs of like kind arise from a growing-point in continual repetition, 

 this occurs generally, though not without exception, in such a way that the youngest 

 organ always stands nearer to the apex of the growing-point than any older organ of 

 the same kind. It follows from this that when we proceed from the base of a shoot 

 and follow organs of like kind — leaves for example — ascending in series on the 

 shoot-axis, and necessarily running spirally around the latter, or ascending it in a 

 zigzag manner, this sequence in space also represents the sequence in time or age in 

 the production of the organs concerned. Such a sequence in the production of 

 organs of like kind is usually termed acropetal. This is to be observed particularly 

 clearly in the case of ordinary foliage-shoots, where the leaves arise without exception 

 acropetally, as may be demonstrated especially in the case of a young, elongating 

 shoot, even without microscopic investigation. The numerous lateral roots arising 

 from a root, again, usually arise in acropetal order, and the same is the case when 

 numerous roots are produced 

 close beneath the growing- 

 point of a shoot, as is the case, 

 for example, in many Ferns. 



The importance of the 

 growing-point lies, as has al- 

 ready been mentioned, in that 

 it consists of embryonic tissue 

 and gives rise to new organs. 

 Confining ourselves to this es- 

 sential character, the growings 

 point need not always be situ- 

 ated at the end of a root or of 

 a shoot, but, on the contrary, 

 a portion of embryonic tissue 

 may be intercalated between 

 masses of permanent tissue. One of these rarer cases has already been 

 noticed on p. 70, Fig. 66; and another above in the formation of a Fig, where 

 the true growing-point at the base of the hollow structure loses its function as 

 such, the further growth of the receptacle and the production of leaves and flowers 

 in its interior taking place by means of an annular zone of embryonic tissue. 

 Even in such cases the organs further distant from the zone of embryonic tissue 

 are older than those next it — in fact, a similar relation exists to that found in 

 ordinary terminal growing-points. This similarity, however, cannot well be expressed 

 by the phrase acropetal succession, and Goebel has therefore, in order to bring out the 

 general fact in the various cases, proposed the term progressive series or sequence. 

 Complicated cases of the progressive formation of organs at growing-points which are 

 not terminal are frequent enough, especially in the development of flowers. For the 

 elucidation of this matter, however, I will employ a monstrosity which I discovered 

 accidentally, and where the fact in question comes out particularly clearly. The 

 accompanying figure (Fig. 304) represents the vertical longitudinal section through 



H h 2 



FIG. 304.— Median lon^tudinal section through a young inflorescence of the Sun- 

 flower {Helianthus amtitits) the growing point s of which had been injured. 



