2o8 On Secondary Thickening in Ptcridophyta. 
ON SECONDARY THICKENING IN RECENT 
PTERIDOPHYTA. 
A Resume. 
F late years evidence of the presence of secondary thickening 
in recent Pteridophyta has accumulated, and it does not 
appear altogether undesirable to gather the recorded facts together. 
In investigating secondary thickening in the plants of a family 
where the process does not usually obtain, some care has to be 
exercised in determining whether the elements which appear to be 
secondary in origin, really are so, and are not instances of a slow 
passage of elements derived from the apical meristem into permanent 
tissue. Of course, in those cases where a well-defined cambium 
is developed, this danger does not exist; a cambium, however, is 
not always present, hence the need for caution. 
To take a possible example ; inmature elements might be found 
in relatively old parts of a member, and may or may not be secondary. 
To determine whether they are of secondary origin or not, evidence 
of tangential divisions are to be sought for, and careful comparison 
made with the structure obtaining in the younger parts of the same 
member, where the tissues are still in a more or less merismatic 
condition. If no evidence of such a division can be found in any of 
the regions of the older parts under consideration, then the case 
would be what has been described above as a slow passage of 
elements, derived from the apical meristem, into permanent tissue. 
It is almost unnecessary to say that in all the instances of 
secondary thickening considered below, the requisite care has been 
exercised, and, in the mind of the present writer, there is no doubt 
that the records are those of true secondary thickening, although 
often of a very rudimentary nature. 
In the majority of cases, xylem is the chief secondary product, 
and the later-formed elements of this tissue may be distinguished 
by the following means:— 
(i.) Position. The secondary elements of the wood are usually 
situated on the outside of the primary xylem, and in the older parts 
of the plants. The age of the member is of importance, and can 
be more or less accurately determined by the number of roots or 
of leaves, in the case of a stem for example ; and by the habits of 
the plant, more especially whether it be slow-growing or not. 
