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R. Ruggles Gates. 
Other writers have carried the principle of recapitulation much 
farther, and notably Jeffrey (1917), who applies it to the histological 
structures of vascular tissues, and further finds that regeneration 
after wounding frequently leads to the reappearance of ancestral 
characters. This principle is freely used by him in the comparison 
of living witli fossil forms, and in the interpretation of phylogenies. 
To cite examples from the Araucarian conifers, it has long been 
known (Thistleton-Dyer 1901) that in the adult stem of these trees, 
owing to the continued activity of a cambium the leaf traces have 
the peculiarity of being persistent for many years after the leaves 
have fallen. Another well known peculiarity of Araucarian wood 
is that the bordered pits in the xylem tracheids are alternate in 
origin, not opposite as in other living conifers. Jeffrey (l.c., p. 236) 
points out that in Mesozoic araucarian woods belonging to the 
genus Brachyoxylon the leaf traces persist only for a short time, 
and the bordered pits are not alternating and crowded as in the 
living genera. In seedlings of the modern Agathis and Araucaria, 
however, “ the leaf trace persists only so long as it is related to a 
functional leaf,” and the pitting is like that found in the Cretaceous 
Brachyoxylon. Such a striking case of recapitulation—and others 
of like nature are known—can hardly find its explanation in a 
germinal change which belongs equally to every cell. 
While these principles of recapitulation seem for the most part 
well established, caution must of course be used in their application, 
especially in these more complicated cases; for it would be easy to 
deduce incorrect phylogenetic conclusions by attaching more 
significance to such cases than they really possess. It is important 
also not to lose sight of the fact that recapitulation phenomena 
occur where there has been adaptation to new conditions. Such 
changes are often climatic, but may also be environmental in the 
widest sense. 
One instance of more doubtful recapitulatory phenomena in 
plants must suffice. In a recent paper on Rhododendron seedlings, 
Professor Balfour (1917) has shown that many of the species in 
their earlier years pass through a series of changes in the 
pigmentation and pubescence of their leaves. Thus in R. adenogynum 
Diels the under surface of the leaves is red glandular in the seedling. 
But about the third year the redness disappears as well as the 
glandular hairs. After some seven years the buff-coloured 
tomentum of long, interwoven, branched hairs begins to show at 
the base of the (now green) leaves, and gradually in later years 
