2 i 4 On Secondary Thickening in Ptcridophyta. 
A few cases have been described in which there was produced 
a new cambium internal to the first. In these cases the new 
meristem arise in the parenchyma—secondary in origin—a little 
towards the exterior of the primary xylem. This cambium behaves 
in a precisely similar fashion to the pre-existing one, giving origin to 
new parenchyma, phloem and wood on its inner side. 
It is obvious that the secondary thickening is, in this plant, 
very variable. Taking its most constant features, as regards the 
locality of the cambium and the direction of formation of its 
secondary products, as the normal, it is more like the process 
obtaining in Dracaena, a fact pointed out by the earlier observers. 
The amount of secondary xylem produced is very variable; 
these tracheides are always present however and they agree in all 
essentials with the lignified elements of the primary wood. 
There remains to be mentioned the slow downward growth of 
the stem, a feature which has been made much of in the past. This 
downward growth is due to the activity of the cambium, which at 
the base of the stem appears to form parenchyma only. This is a 
point of some importance and shows that there is no real homology 
with the apical growth of a primary root. 
It is not proposed to enter into any elaborate discussion 
regarding the significance of the presence of secondary thickening 
these plants. Attention, however, may be drawn to the opinion 
that, on the whole, the recent Ptcridophyta exhibit considerable 
reduction. A large number of the fossil Ptcridophyta possessed 
marked powers of secondary growth in thickness, and therefore it 
is not a matter for surprise to find that certain of their descendants 
still retain traces, more or less well-marked, of this capability. For 
example the obvious connexion between Equisetum and Calamites 
may be cited. 
For these reasons the secondary tissues in the recent forms 
are to be looked upon as instances of reduction rather than cases 
of the inception of new developments. 
T.G.H. 
