388 Farmer and Hill.— -Arrangement and Structure 
the stem grows and becomes larger, and the vascular system 
more complicated, the commissural strands gain in size, and 
completely resemble in structure that of the larger strands 
of the siphonostele forming the margins of the foliar 
gaps. 
Allusion has several times been made to the addition of 
fresh elements to the tracheidal mass of the stele. A careful 
comparison with corresponding parts of the lower region of 
the stem in sporophytes of different ages suffices to awaken 
the suspicion that a secondary addition occurs in consequence 
of merismatic activity in the parenchyma outside the xylem h 
That such is the fact is easily proved, for the new tracheids 
not only by their position are easily referable to their initial 
cells from which they respectively originated (Figs. 26, 28), but 
the various stages of their development may be traced with 
a certainty. The cells which undergo cambial division are 
commonly situated next the existing xylem, and they may 
occur singly or in groups. We have not observed any instances 
in which the whole parenchyma surrounding the xylem was at 
one time merismatic, but in favourable instances as many as six 
or eight such cells may be detected. The secondary wood 
thus formed is unimportant in amount, seldom exceeding 
three cells in depth, but the existence of secondary thickening 
in the Marattiaceae adds yet another example to those groups 
of Cryptogams in which it has been recognized, and shows 
that its presence is even less limited than has been supposed. 
Traces of it may still be found in larger stems, but in them it 
is of more isolated occurrence as compared with the younger 
plantlets. It is easily seen in Marattia (Fig. 33), but we have 
not been able to observe it in Kaulfussia. 
In addition to this secondary formation of tissue within 
the stele, there is in Angiopteris considerable and very regular 
tangential division (Fig. 23) in the endodermal layer of the stem. 
In this way a number of radially arranged layers of cells are 
formed which may be as many as five to six in number. 
Commonly the outermost cell-row of this tissue assumes 
1 T. G. Hill, Annals of Botany, xvi, p. 173, 
