II. Preliminary note on the apex of the leaf in Osmunda and Todea . (From the Jodrell Laboratory, Royal Gardens, Kew.)

It has long been accepted, in accordance with the investigations of Sadebeck, that there is at the apex of the young leaf of the fern a two-sided, wedge-shaped, apical cell, and that, after this cell has lost its identity by periclinal, and subsequently by anticlinal divisions, the growth of the leaf is continued at the margin by the persistent activity of a linear series of marginal cells. It is true that this is the mode of development of many fern-leaves, but, as my observations show, it does not apply for all cases, while those exceptional cases are particularly interesting as occupying an intermediate position in this, as also in other, respects between the true ferns, on the one hand, and theMarathaceæ andCycadeæ on the other. It is among theOsmundaceæ that these exceptional cases occur. In the young leaves of Todea superba and ofOsmunda cinnamomea it was found that the apex is occupied by a well-marked,three-sided , conical, apical cell, from the three sides of which, segments are cut off in regular succession, as at the apex of the stem ofEquisetum . The apical cell is so placed that one side faces the ventral side of the leaf, while the remaining two sides are obliquely disposed with regard to the dorsal side of the leaf. No clearly marked marginal series of persistently active cells have been found, giving rise to the pinnæ, as is stated to be the case for the typical ferns. Further, there appears to be no strict relation between the points of origin of the pinnæ and the segments cut off from the apical cell. The pinnæ arise in acropetal order. In itself no great importance is to be attached to the difference between a three-sided and a two-sided apical cell. For example, it has been clearly shown in a paper by Treub, on the vegetative organs ofSelaginella Martensii , that the two forms of apical cell are to be found on different shoots of the same species. But in the case of the leaf of the fern, the whole development, as described by Sadebeck and by Ruy, is so closely connected with the‘existence of a two-sided cell that a departure from that arrangement is to be regarded as of more importance than would otherwise be due to it, and it appears to me to supply an intermediate step towards the more complex leaf of theMarathaceæ andCycadeæ .