608 BOTANY PART n 



elongated axis of the flowers of Gymnosperms. The Gnetaceae, which are also 

 treated by WETTSTEIN as a connecting link between Gymnosperms and Angio- 

 sperms, foreshadow in the androgynous inflorescence of Gnetum a flower like those 

 of the Polycarpicae. Further, the Calycanthaceae, which are placed in the latter 

 group, have an extensive sporogenous tissue in the nucellus such as is only known 

 in some Gymnosperms, in Casuarina, and in Rosaceae, a family that is to be 

 connected with the Calycanthaceae. 



The evidence for this second possible line of progression renders it 

 as probable as the one first mentioned. Both regard the Gnetaceae 

 as a transition family, and it is thus conceivable that both lines of 

 development have been followed in plant-evolution. The less highly 



organised Monochlamydeae would 

 come in the manner indicated by 

 WETTSTEIN from Ephedra to Dico- 

 tyledons ; the Dialypetalae in the 



^y . \ second way from Gnetum to the 



Polycarpicae. So long as develop- 

 mental and morphological evidence 

 is insufficient to establish a common 

 origin of the two sets of Dicotyle- 

 ; <lons and their connection as sug- 



gested by HALLTER in the Hama- 

 melideae, this double origin appears 

 most probable. 



SUB-CLASS I 

 Dicotylae 



^ The Dicotyledons with few ex- 



ceptions possess a pair of seed- 

 leaves. The distinction of hypogeal 

 and epigeal germination has been 



FIG. 601,-Leaf of Crataegus with reticulate ^Scribed On p. 589. _ 



venation, (f nat. size. After NOLL.) The stem has a circle of open 



vascular bundles, while the root 



on transverse section shows a regularly alternating arrangement of 

 the xylem- and phloem-groups (cf. p. 136, Fig. 163). The meristem 

 situated in the vascular bundles of the stem, or to the inner side of 

 the phloem in the root, soon becomes completed across the medullary 

 rays and forms a complete, meristematic ring. By means of this 

 cambium a regular growth in thickness of the stem and root takes 

 place. 



The typical form of leaf found among Dicotyledons is provided 

 with a longer or shorter petiole, and often has a pair of stipules 

 developed from the leaf-base ; a leaf-sheath is usually absent. The 

 lamina may be simple or compound ; the latter condition is always 





