knowledge of the Anatomy of Plants . 153 
them. Hence the new elements simply compress and ob- 
literate the old. The same is the case with medullary phloem 
whenever it has cambial increase, and further illustrations may 
be drawn from the anomalous bundles, imbedded in sclerotic 
tissue of the Chenopodiaceae and allied families. Again, in 
the vine, for every year’s new growth of phloem the produce 
of a previous year is thrown off. As regards mere increase 
in quantity of tissue there is in all such cases a great waste. 
I believe the explanation is to be sought in the principle thus 
formulated by Weismann (p. % 1, English edition):- — ‘ The 
vital processes of the higher animals are accompanied by 
a renewal of the morphological elements in most tissues/ 
So I believe it is with the most highly organized plants in 
the majority of classes, and especially the Dicotyledons and 
Gymnosperms. The renewal of tissue, the replacement of 
effete, overworked elements by new and energetic ones, is an 
end in itself, apart from the mere increase of tissues in bulk. 
The principle is the same as in the more obvious case of the 
replacement of old leaves by fresh ones. 
If is remarkable for how long a time the same elements 
must continue functional in the stem of trees (such as palms) 
without secondary growth. This is really an exceptional 
case among organic beings. 
The subject of anomalous thickening in Dicotyledons suffers 
in interest from our bald and meagre knowledge, especially of 
the development. The study of such profoundly modified 
structures in the culminating class of the vegetable kingdom 
must lead eventually to interesting results. We already have 
real progress to note. 
In a group of families including the Chenopodiaceae, Nycta- 
gineae, and Amaranthaceae we have a conspicuous anomaly 
consisting in the appearance of extra-fascicular cambium,, 
either in the place of or in addition to the normal layer. 
This extra-fascicular zone may either be permanently active, 
producing ring after ring of collateral bundles, or its function 
may cease after one such ring has been formed, its place 
being taken by a new zone of cambium arising once more 
