440 BOTANICAL GAZETTE [JUNE 
SECONDARY TISSUES. 
Tissues derived from the cambium ring. 
The tissues of the bark, regularly derived from the cambium ring, 
include the secondary sclerenchyma and the secondary phloem. In 
addition to these, scattered groups of stone cells, which should prob- 
ably be considered a part of the phloem, also make their appearance. 
Of course the cambium also adds new elements to the primary medul- 
lary rays and brings about the development of the secondary rays 
(figs. 1, 5). The development of these various secondary tissues 
begins during the first vegetative period. 
The fibers of the secondary bast do not form bundles. Some of 
them form interrupted layers arranged concentrically in the stem, 
others are scattered through the secondary phloem. The layers ar? 
usually but a single cell across and are separated from one another 
by several layers of phloem. The individual fibers are rectangular 
in cross section and about thirty times as long as broad; their walls 
are very strongly thickened (jigs. 1, 5, 6, sc,). When the bast fibers 
are cut off by cork all regularity in their arrangement disappears. 
The sieve tubes of the secondary phloem, except those earliest 
formed, are arranged in interrupted, concentric layers, one or two 
cells across (fig. 5, ph,). Many of the sieve tubes are in direct con- 
tact with the medullary rays, but very few of them adjoin the scleren- 
chyma fibers. The tubes exhibit essentially the same structure ‘as 
those in the primary phloem. On account of their delicate walls they 
become practically indistinguishable when cut off by cork. 
The bulk of the secondary phloem is composed of parenchyma. 
When first differentiated from the cambium the cells of this tissue 
are closely packed together, rectangular in outline, and destitute of 
intercellular spaces. As they become pushed outward, their outlines 
become more rounded and minute intercellular spaces appear. Their 
walls are fairly thin but are provided with simple pits. Until they are 
cut off by cork the parenchyma cells are arranged in layers, which lie 
among the layers of sclerenchyma and sieve tubes. 
The groups of stone cells are irregularly scattered in the secondary 
bast but always abut against a medullary ray (fig. 5, sé). Such a 
group in cross section is often larger than a bundle of primary scle- 
renchyma and is composed of larger elements. The stone cells are the 
