in the Root and Stem of Dicotyledons . 285 
amounts to twenty-five elements, equal to about one-eighth of 
the normal wood. The internal cambium forms most phloem 
where it does not produce any wood. Opposite the woody 
groups the number of phloem-elements is smaller and they 
also undergo greater compression (see Fig. 14). 
The internal centripetal wood, where it is well developed, 
consists of elements of three kinds: (1) fibrous cells with 
pointed ends which form the small lumina seen between the 
larger elements in transverse sections (Fig. 14) ; (2) xylem- 
parenchyma, and (3) ray-parenchyma. The internal xylem- 
rays resemble those of the normal wood, but their cells are 
less elongated radially. All the elements have lignified walls, 
and simple elliptical pits, which in the fibrous cells are 
spirally arranged. The internal wood chiefly differs from the 
external in the absence of vessels. 
It may be added that the internal sieve-tubes are of 
perfectly typical structure, with lateral sieve-fields as well as 
transverse plates, which form callus in autumn. 
The pith is very lacunar and offers comparatively little 
resistance to the growth of the internal vascular tissues. 
2. Willughheia firma , Bl. (Apocynaceae). This climbing 
representative of the order shows the same structure as 
Apocynum cctnnabinum , namely centripetal internal wood, but 
in a still more striking form, for in Willughbeia the internal 
wood contains large vessels. Its development begins here 
even later than in Apocynum. When the normal wood is on 
the average about eighty cells in radial thickness there is still 
no sign of internal xylem. The inner phloem-groups form a 
ring, and are only separated from one another by narrow rays 
of parenchyma. There is already an active internal cambium 
which has arisen by division of about the third layer of 
parenchymatous cells within the protoxylem, and has added 
largely to the phloem, so that the older phloem-elements and 
some of the pith-cells also are becoming crushed and obliter- 
ated. The pith contains very large thick-walled sclerenchy- 
matous fibres, and also large laticiferous cells. As the inward 
growth of the internal phloem continues, the rays keep pace 
