“With regard to your method of explaining the fundamental characters of ‘Exogens’ and ‘Endogens,’ this of course is part of the same hypothesis; but I may point out that since Von Mohl and Sanio, between 1855 and 1865, showed (1) that the growth at the stem apex of a monocotyledon was _not_ endogenous, and (2) that the ‘thickening ring’ near the apex of a dicotyledon was not to be confused, as had been done up till then, with the ring of _secondary meristem_ or _true cambium_, which arose lower down, and only in woody or practically woody stem, the terms ‘Exogen’ and ‘Endogen’ have necessarily fallen into disuse, since they imply a false conception of what happens. Both monocotyledons and dicotyledons have a ‘thickening ring,’ which gives rise to the primary vascular cylinder of the stem. When the stem is of considerable thickness, as in Palms, &c., it grows by the active cell-division of its outer layers, so that both classes are ‘exogenous’ in this sense; while the addition of a centrifugal zone of secondary wood is confined to certain Dicotyledons (Trees, shrubs, &c.). “The distinction between the embryos, moreover, is not absolute. The single cotyledon is usually terminal in monocotyledons, but not always (_Dioscoraceæ_ have lateral cotyledons), but the plumule may push through it (Grasses) or make its exit sideways (Palms), or be formed at the side (_Alisma_); and Dicotyledons very similarly. “The occurrence of completely sheathing leaves in grasses is perhaps correlated with the absence of cambium, but grasses are an aberrant type among monocotyledons, and secondary thickening is only found in very few genera of this class, so that the correlation is, so to speak, negative and indirect.... It is clear that the greater part of the discussion will have to be re-written.” For the reasons assigned in the preface I cannot undertake to re-write the discussion, as suggested. It must stand for what it is worth. All I can do is here to include along with it the foregoing criticisms.