806 THE AMERICAN NATURALIST. (VoL. XXXIX. 
A comparison of these evolutionary phases with mos PURT 
determined for Cordaites (Penhallow, :00, p. 57, and ; nn 
250) will show that thetwo are essentially parallel, nn. TE 
one another only in detail in the two directions alrea y at d 
So far then as the Salicacez are concerned, there is abun an 
supplementary evidence to show that in that family e és = $ 
the gymnosperms, the evolution of the secondary xy des BER 
protoxylem is determined by the special requirements o iae i 
and the movement of the transpiration current ; that the fibrous, 
non-pitted wood cells serving the ends of mechanical EE 
derived from the spiral tracheids in the first instance rx à; 
the relative predominance of mechanical or conductive tissue, : 
the resultant of influences acting along two distinct lines o 
ment. 
m, Cells.—Very little diagnostic value attaches pes 
appearance or disposition of the wood cells as presen = 
transverse section, and such features as they do exhibit are rather 
of ordinal than of generic or specific value. They are mre 
hexagonal and very variable as to size. Usually they are ees 
posed in a very irregular manner, though occasionally there is a 
more or less pronounced tendency to disposition in radial rows, 
but in no case is this so pronounced as in the Coniferae — a poy 
ence which may be said to distinguish the gymnosperms from al 
the higher types of woody plants. The walls are of medium 
thickness, a feature which seems to be consistent with the gen- 
erally soft character of the woody structure, and, as already 
pointed out, there is a complete absence of that unequal, regional, 
secondary growth of the cell wall which, in other woods, con- 
tributes so largely to a differentiation of the spring and summer 
woods. On the whole, the structure of the mechanical tissue is 
closely comparable with that of the Rhamnaceæ, a resemblance 
which will be found to extend to other features of the vascular 
zone. 
Wood Parench yma. — In 
parenchyma varies considerab 
Structure. Within the re 
ments are more or less 
vessels, the tendency is to 
its transverse section, the wood 
ly according to its position in the 
gion of the medullary sheath the ele- 
isodiametric ; when associated with 
compression conformably with the 
* 
