CHAP, v.] RELATIONSHIP AMONGST PLANTS. 159 



other writers, professes to take into consideration all 

 important characters in formulating the lines of true 

 relationship or descent, yet in reality the most advanced 

 natural systems derive most of their important characters 

 from the reproductive phase, and more especially those 

 furnished by the flower and the fruit, which necessarily 

 includes the seed. The structures present in a typical 

 flower have been already described and illustrated in a 

 diagrammatic manner in Fig. 7 ; the actual necessity 

 for the presence of a flower, as understood in Phanero- 

 gams, resulting from the compulsory change in the mode 

 of fertilization when plants established themselves on 

 dry land, has also been discussed, and it now remains 

 to show, that in spite of the distinct structural characters 

 presented by flowers, their componont parts are not new 

 additions to those possessed by cryptogams, but merely 

 modifications of previously-existing structures, in fact it 

 may be truly said that the reproductive portion of a 

 phanerogam is nothing more than a changed portion of 

 its older vegetative structure. The vegetative portion 

 of a plant developed above ground, leaving out of ques- 

 tion for the moment certain minor features, consists of a 

 stem bearing leaves, the latter being scattered or sepa- 

 rated from each other by internodes, a feature, from the 

 vegetative point of view, of prime importance, inasmuch 

 as it enables the individual leaves to secure the required 

 amount of exposure to light, a point on which the per- 

 formance of their most important function, that of 

 assimilation, depends. Leaves do not appear hap-hazard 

 on a twig, but develop according to a definite law, 

 known as acropetal development, which means that the 

 oldest leaf is situated at the base or oldest portion of the 



