HISTOLOGY OF PHLOEM IN WOODY ANGIOSPERMS 365 
or completely crushed by, sclerenchyma. Fibers may be scattered 
singly throughout the phloem as in Cephalanthus (fig. 40), arranged in 
regular tangential bands as in Salix and Tilia (fig. 39), or placed in 
irregular groups. They may be fairly straight, as is usually the case, 
or may be very much contorted and twisted as in Acer rubrum. Stone 
cells may occur singly or, as is more common, may be grouped in 
irregular masses. Both fibers and stone cells are frequently grouped 
together in various ways. The distribution of sclerenchyma is distinct 
for a given species, and within a species is fairly constant as far as the 
number of collections concerned in this study show. 
The origin of fibers and stone cells in the phloem seems to be very 
different in different plants. In many cases fibers are cut off by the 
cambium in regular tangential bands and therefore appear near the 
cambium. Just as frequently, however, sclerenchyma is not present 
except in the outer phloem, in which case it is formed either by the 
lignification of existing parenchyma cells formed by the cambium, the 
proliferation of parenchyma and its subsequent lignification, or pos- 
sibly by the formation of rows of sclerenchyma by a layer of cells 
comparable to a phellogen layer. Frequently the medullary rays are 
lignified along with the masses of parenchyma through which they pass. 
Crystals are of very frequent occurrence in the phloem of many 
plants. Probably the rhomboidal type is the most common, occurring 
abundantly within stone cells and septate fibers and also in different 
types of parenchyma. Druses are frequent but usually within paren- 
chyma cells rather than sclerenchyma. The type and distribution of 
crystals, however, seems to vary with ecological conditions and rapidity 
of growth. 
The parenchyma in the phloem of the mature woody plants studied 
is rather surprisingly uniform, the divided-cambiform type of Haber- 
landt (4) or modifications of it being of almost universal occurrence. 
The true cambiform type of cell with prosenchymatous form was not 
found in any of the plants studied, and the so-called conducting type 
was found in only a few, e. g., Tilia and Cephalanthus. In fact, from 
the material studied, it seems practically impossible to make a good 
distinction between the cambiform and the conducting types because 
of the intergrading forms, a point brought out by Strasburger (17). 
In the development of secondary phloem parenchyma, an elongate 
cambium cell (figs. 10, 31) divides longitudinally to produce a paren- 
chyma mother cell, which in turn divides transversely to form the 
