North American Sa/icales. 
171 
horizontal and vertical planes they become the multiseriate rays charac- 
teristic of the mature wood of most Dicotyledons . 1 This process can be 
observed especially well in certain of the Ericaceae and Casuarinaceae, where 
the compound ray is present only near the leaf-trace. In fact, it has been 
shown that the leaf-trace has played the chief part in the formation of 
compound rays . 2 As these multiseriate rays go from the pith, they are 
apt to dwindle, as seen in the seedling of Populus deltoides , for example, 
until uniseriate rays are formed. This is the condition in the Salicaceae, 
Aesculus , and, as Mr. Bailey has ascertained, in Castanea , all of which have 
uniseriate rays normally in the adult, but multiseriate rays in the primitive 
regions. 
Whether the direct ancestors of the Salicales ever attained compound 
rays like those of the oak, it is impossible to say. All that comparative 
anatomical evidence shows is that they are descended from ancestors which 
possess multiseriate rays, and adequate palaeobotanical evidence on their 
origin is lacking at present. It is evident, however, that the compounding 
process must have taken place in very remote geological time, since multi- 
seriate rayed forms are common in the Upper Cretaceous of Japan . 3 
The development of parenchyma is somewhat as follows : In the 
Abietineae there is no parenchyma in Pinus ; but in all the other members 
there is terminal parenchyma. In the Sequoineae and Cupressineae the 
parenchyma is diffuse, i. e. scattered through the year’s growth. Since 
the Gymnosperms never developed vessels the parenchyma never got 
beyond the diffuse condition. It is likewise diffuse in the Gnetales and 
in the lower Angiosperms, as the Casuarinaceae, Betulaceae, Fagaceae, 
and Juglandaceae ; while in the highest Angiosperms it is clustered around 
the vessels, as in the Oleaceae, Ulmaceae, Leguminosae, & c. Reduction 
then sets in and the Saliceae and Magnoliaceae have terminal parenchyma 
in the stem, but vasicentric in primitive regions. It is of interest to note 
that conditions in the Magnoliaceae are similar to those in the Salicaceae — 
the Asiatic genus, Michelia , has vasicentric parenchyma in the stem, and 
Liriodendron tidipifera has vasicentric parenchyma in traumatic tissue. 
Another interesting case of reversion is seen in Osmanthus , a member of 
the Oleaceae, where the parenchyma is exclusively terminal. 
From this consideration of the evolutionary tendencies in the develop- 
ment of rays and parenchyma, it is evident that the Salicaceae, instead of 
coming low down among the Dicotyledons, as is assumed by systematic 
botanists, in reality represent, from the anatomical standpoint at least, 
1 Thompson, W. P. : The Origin of Multiseriate Rays in Dicotyledons. Annals of Botany, 
vol. xxv, 1911, p. 1005. 
2 Bailey, I. W. : The Relation of the Leaf- trace to Compound Rays in the Lower Dicotyledons. 
Annals of Botany, vol. xxv, No. xcvii. 
3 Stopes and Fujii : Studies on the Structure and Affinities of Cretaceous Plants. Phil. Trans- 
actions of Royal Society of London, Series B, vol. cci. 
