160 BOTANY. [chap. v. 



twigj and the youngest nearest the tip or growing-point. 

 A young leaf never appears on a twig below, or lower 

 down, than another leaf that is already developed, and 

 when it is stated that the parts of a flower are modified 

 leaves, what is meant in reality is that the various whorls 

 of the flower follow the same acropetal law of develop- 

 ment as foliar organs or leaves, and not that sepals, 

 petals, stamens and carpels are in the young condition 

 leaves that afterwards change their functions. The 

 reversion of all the parts of the flower to foliar or 

 leaf-like organs, a not uncommon occurrence, supports 

 the above view. As in the case of foliage leaves it has 

 been shown to be an advantage that these should be 

 separated from each other by internodes, so in the case 

 of the flower it is a decided advantage that all the parts 

 should be concentrated ; this is efifected by the suppres- 

 sion or non-development of the internodes of the stem, 

 and although this is in most flowers and inflorescences 

 the case, yet careful attention to the order of develop- 

 ment shows signs of the ancient idea of acropetal 

 development. If an expanding sunflower inflorescence 

 is watched it will be noticed that the ray florets do not 

 expand simultaneously all round, but that expansion 

 commences at one point and gradually works round to 

 that point, the first florets to expand occupying a position 

 in the concentrated arrangement corresponding to the 

 lowest and oldest leaYes on an elongated stem, while 

 those that are latest in expanding correspond to the 

 youngest leaves on a lengthened twig. The same idea 

 is observed in the development of the various parts of 

 many flowers; the sepals, petals, and stamens, that 

 respectively appear to grow in whorls at the same level 



