26 The Botanical Gazette. (January, 
three kinds of tubercles. Those of a yellowish tan color, 
generally found above the surface, are devoid of the symbio- 
tic algz; the second variety, of a slightly darker tan and 
often greenish near the tip, always contain the alge; they are 
the younger tubercles. The third variety, which also contain 
the alga, are of a dark brown color and are older than the 
others. These three varieties are found on the same plant. 
In external appearance they resemble somewhat the tubercles 
of Pisum sativum. Morphologically they differ considerably. 
In pea tubercles the symbionts are surrounded by the vascular 
bundles while in Cycas the symbionts surround the cen- 
trally located vascular system. They resemble each other 
in their mode of branching. 
Their mode of development is quite simple. Either the 
ends of rootlets branch dichotomously or they develop endo- 
genously. It may be more correct to say that all tubercles 
develop like primary roots and that the lateral development 
is only apparent. That is the developing lateral root branch 
receives its tubercular peculiarity from the very start. The 
line of demarcation between rootlet and tubercle is very dis- 
tinct andabrupt. The tubercle branch has about three or four 
times the dimensions of a rootlet of the same length. 
On making a cross section of any part of the tubercle ex- 
cepting the tip one can see with the naked eye a green circu- 
lar layer about midway between the epidermis and bundle 
sheath. This is the alga-bearing layer. At certain points 
this green layer is discontinuous. This always occurs op- 
posite outer lenticular structures which are quite common on 
the tubercles and are arranged in more or less broken rings. 
Having thus treated of the gross anatomy I shall next de-- 
scribe the minute anatomy. q 
A cross section shows six tissue layers. The first and - 
outermost is the dermal layer of irregular corky cells several - 
rows in thickness developed from a dermatogenic layel 
dividing tangentially. In the dermal layer are also included — 
the lenticular structures consisting of enlarged irregular corky 
cells which do not seem to develop from any definite phel- 
logenic layer. The cell walls of the dermal layer give to the | 
tubercle its yellowish or dark browncolor. The cells contain, — 
besides the remains of nuclei and cytoplasm, various kinds of — 
rhizobia in comparatively small numbers. The entire surface 
of rootlets, roots and tubercles is more or less covered by 
