468 LECTURE XXVIII. 



a young flower head of the Sunflower {Helianthus annuus). Its true growing-point 

 was situated in the middle of the disc n n, but the central apex of this growing-point had 

 been damaged by some means : the young tissue in its immediate neighbourhood 

 had lost the nature of a growing-point, but had become elevated in the form of 

 a protuberance as a, while at the base of the latter z 0, a zone of embryonic tissue had 

 been formed. The rudiments of new flowers and the bracts belonging to them only 

 arise now on this zone. So long as the central apex of the flowering head was yet 

 living, the flowers and bracts were produced on the disc in acropetal succession n n, 

 or, as we may say in this case, in centripetal order: the flowers next the circum- 

 ference are the oldest, those nearest the centre the youngest. After the destruction 

 of the central apex, as the protuberance a s a was elevated from the zone 2 z, 

 however, the previous centre or apex behaved exactly as if it were the oldest part of 

 the protuberance : the production of flowers proceeded from the point j of the protu- 

 berance successively towards the zone z z, and we perceive that at the circumference of 



Fig. 305.— Diagram to illustrate the mode in which leaves are developed from the growing-point of a Phanerogam. 



this zone the youngest normally formed flower-rudiments are situated, as well as the 

 youngest abnormally formed ones. The arrangement in both cases is still more 

 clearly manifested by the relative position of the bracts and the flowers which 

 belong to them : it is noticed that on the normal portion of the inflorescence each bract 

 is situated on the outer side of the flower which belongs to it ; on the abnormal portion 

 as a, on the contrary, the bracts are situated nearer to the former apex i than the 

 flowers which belong to them. To come back now to the starting-point of our 

 consideration, we may thus say that the formation of organs before and after the 

 destruction of the central apex has occurred in progressive order ; that is, progressive 

 towards the true apex before the injury, and progressive towards the embryonal 

 zone z z after the injury : and it is clear that it v/ould not be quite to the point to 

 designate the sequence of the formation of organs on the protuberance a .fa as 

 acropetal. 



The acropetal or progressive order of development is strictly maintained by the 

 leaves on ordinary vegetative shoots; at least no exception in this connection is as 

 yet known. In the region where flowers are formed, on the contrary, cases occur as a 



