210 On Secondary Thickening in Pteridophyta. 
The greatest addition of new tracheides is to be found in the 
bases of the roots, which fact corresponds with what has been found 
to obtain in the roots of Botrychium. 
MARATTIACEAE. 
Literature. 
Farmer, J. B., and Hill, T.G.—On the Arrangement and Struc¬ 
ture of the Vascular Strands in Angiopteris evecta and some 
other Marattiaceae. Ann. Bot., Vol. XVI., 1902. 
Hill, T. G.—On secondary Thickening in Angiopteris evecta. 
Ann. Bot., Vol. XVI., 1902. 
Angioptekis evecta. 
Stem. The secondary thickening which occurs in this plant, is 
of a somewhat more definite nature than what obtains in the 
Ophioglossaceae, for there is a fairly well-defined cambium formed 
by the division of the parenchymatous tissue surrounding the xylem. 
These merismatic cells do not extend around the whole of a 
vascular strand, but are local in their occurrence ; in the best 
examples, from six to eight such elements may be observed. The 
amount of xylem produced is small, being rarely of more than 
three cells in depth and this only in the best instances ; the more 
usual thing to find is the occurrence of single tracheides. 
Besides this intrastelar secondary tissue-formation, there is 
also a very regular cambial division in the endodermal layer. This 
activity leads to the formation of sometimes as many as six layers 
of cells arranged in a radial manner; the products of the division 
remain parenchymatous, but the outermost elements may faintly 
respond to the test for endodermis. 
Leaf. Occasionally there may be seen secondary tracheides 
bordering the xylem of the leaf-trace at the base of the petiole. 
No cambium has been observed, there is merely an addition of 
single elements. 
Root. No observations regarding secondary thickening have 
been recorded. 
Marattia fraxinea. 
Stem. The process is of exactly the same nature as has been 
described for Angiopteris. There is also, in Marattia , the same 
thickening of the parenchyma bordering on the stele, but not to so 
great an extent as in Angiopteris. 
As regards the other members of the Marattiaceae, no addition 
of secondary elements seemingly takes place in ArcJiangiopteris, 
Danaea or Kaulfussia. 
