APPENDIX A 505 



DICOTYLEDONEAE. 



These Plants are characterised by the embryo bearing two coty- 

 ledons. The leaves are net-veined, usually with a narrow base, 

 and a definite petiole. The stem and root show secondary thicken- 

 ing by means of a cambium. The flowers are usually pentamerous, 

 or tetramerous, with distinct calyx and corolla. The plants are 

 perennial or annual, many of the former developing as shrubs or 

 trees. 



The Dicotyledons are divided into two large series, according to 

 the separateness or coherence of their petals. This distinction does 

 not serve a like purpose in the classification of the Monocotyledons ; 

 it has already been seen that the very natural Family of the Lilia- 

 ceae is variable in this respect (p. 495). But in the Dicotyledons 

 the same variability within natural families is exceptional : there- 

 fore this distinction serves to give a natural separation of them into 

 Polypetals, or Choripetalae, with the petals all separate from one 

 another ; and the Gamopetals, or Sympetalae, where there is a coher- 

 ence of the petals to form a united, usually tubular corolla. 



The former is undoubtedly the more primitive state. It repeats 

 the condition usual in the vegetative region, and it is characteristic 

 of those less specialised flowers which on many other grounds are 

 held as less advanced. The gamopetalous state is characteristic 

 of flowers which are more specialised as pollinating machines, and 

 they may therefore be held as more advanced. But there is no 

 reason to hold all plants showing gamopetaly as necessarily related to 

 one another : this would involve the assumption that this advance 

 had happened only once in the course of Evolution. It seems prob- 

 able that in a plurality of evolutionary lines the advance was made to 

 gamopetaly, and the student should be prepared to recognise this in 

 any sequence in which comparison makes it appear probable. In 

 accordance with the views thus briefly sketched the Polypetals, or 

 Dicotyledoneae Choripetalae, will be taken first. 



DICOTYLEDONEAE CHORIPETALAE. 



ORDER: SALICALES. 



Family : SALICACEAE. Example : THE GOAT WILLOW. 



(12) The numerous native species of Willow are trees, or shrubs, or dwarf 

 undershrubs, which live in damp situations ; ahnost any of them would serve to 



