544 
Reed.—On the Anatomy of some Titbers. 
of three or four vessels each, with intervening pockets of parenchyma. A 
transverse section of a portion of a tuber at this stage is shown in 
PI. XLIII, Fig. 14, where, as a result of the activity of the xylem parenchyma, 
the xylem appears to be broken up into islands (xy.). Strictly speaking, 
these are not true vascular islands, for in longitudinal section they are seen 
to be continuous, and not pinched off, as they would be were they true dots 
vasculaires . The following diagram shows a longitudinal section of such 
a tuber:— 
Diagram III. xy. par., xylem parenchyma. P.xy., protoxylem. xy., xylem. J\I., pith. 
A series of transverse sections passing from A to B would present the 
features as shown in Diagram IV. 
In the above description the term dilatation parenchyma has been 
purposely avoided. We have no evidence to show that the xylem paren¬ 
chyma in this case is true dilatation parenchyma. This term is restricted to 
parenchyma which has ordinarily ceased to be active, but which may be re¬ 
called to activity by pathological conditions caused by fungal attacks, 
mechanical injury, &c. 
At the stage represented by Fig. 9, PI. XLII, the cambium does not 
extend the whole way round the stem, but presently this extension is 
effected. The fascicular cambium produces very few new lignified elements, 
in which respect it agrees with X. tuberosum ; most of the tissue to which 
it gives rise is xylem parenchyma which goes to form part of the tuberous 
tissue. The interfascicular cambium gives rise to no new xylem vessels, but 
restricts itself to the production of parenchymatous cells. The broad paren¬ 
chymatous medullary ray tissue thus produced is arranged in regular radial 
rows as shown in PI. XLII, Fig. 11, M.ry., and contributes largely to the 
tuberous tissue. A similar tissue is described as occurring in tubers of 
Euphorbia Intisy by MM. Dubard and Viguier ( 5 ). 
