THE ORIGIN OF ANGIOSPERMS. 41 
Sali; is primitively unisexual, diœcious, and naked. Robertson *, on general 
grounds, considers Populus the more recent genus. 
On the other hand, the opposite view has been maintained, and has quite 
recently received strong support from Haines’ t deseriptions of two new 
species of Indian Poplars. Опе of these, Populus glauca, Haines, frequently 
possesses hermaphrodite flowers with an undoubted perianth. 
To us it appears to be the more reasonable view to regard Populus as the 
older genus, and Salix as derived from a Poplar-like ancestor at a more recent 
period. On this view, Populus glauca has retained more primitive features 
than the other species, and thus departs less from the ancestral type, while 
the entomophily of Saliw is but a recently acquired character. It also implies 
the derivation of a flower such as that of Saliw, which has only two stamens 
and never more than two carpels, by reduction from a plant like Populus, 
possessing many stamens and sometimes more than two carpels. At the 
same time, we regard Populus itself as not primitively anemophilous, but 
derived originally, with the other Amentiferous families, from entomophilous 
ancestors. 
Another fact to be taken into consideration in this connection is that, 
at the present day, the genus Salix is largely represented by species, whereas 
Populus contains comparatively few. The former thus appears to be a plastic 
up-grade type, as is further emphasized by the ill-defined nature of many of 
its species. The re-adoption of entomophily has possibly been the saving of 
the Willows. 
Casuarina.—For those who uphold the view that some of the present day 
Angiosperms without perianth are primitively naked, this genus has been 
perhaps the most promising. In several features it strongly suggests the 
retention of archaic structures. Attempts have been made to separate it 
entirely from the rest of the Dicotyledons. Treub’s $ suggestion, based on the 
initial discovery of chalazogamy, broke down when the fact was ascertained 
that many of the Amentiferz, as well as other groups, also exhibit this mode of 
fertilization. On the other hand, Engler § regards Casuarina, which he places 
in a new cohort Verticillate, as the most primitive of Dicotyledons, from the 
fact that many megaspores are found within the nucellus. In the light, 
however, of recent research this conclusion is by no means Justified, for 
Chamberlain || finds that more than one megaspore occasionally occurs in 
Salix. Still more recently Shoemaker Ч has shown that in Hamamelis several 
megaspores are found. Frye’s** study of the embryo-sac of this genus has 
demonstrated that it is quite of the usual type, and does not differ as regards 
* Robertson (1904). T Haines (1906). 
1 Treub (1891). $ Engler (1897) p. 362. || Chamberlain (1897). 
$ Shoemaker (1905) ; see also Coulter & Chamberlain (1904) р. 242. 
** Frye (1903). 
