1901] TUBER-LIKE ROOTLETS OF CYCAS 267 
the base of the tubercle almost to the very apex (fig. 3). Where 
first distinguishable it consists of cells whose contents are some- 
what denser than those of the neighboring cells, and before any 
intercellular spaces appear the cells 
next to the zone on both sides become 
very distinctly differentiated into a 
layer (/) resembling a tapetum. When 
fully developed the algal zone consists 
of loosely connected cells with large 
intercellular spaces which are occupied 
by the algae (figs. 4,5). The radial 
elongation of these loosely connected 
cells induced Schneider to call them 
palisade tissue. Where large lenticels Fic. 2.— Transverse section 
occur there is a break in the algal zone, of tubercle. X15. a, algal 
and instead of the usual layers the 
round parenchyma cells extend from the phellogen to the vascu- 
lar cylinder (fig. 6). The round cells with their small inter- 
cellular spaces 
form an effective 
air-conducting tis- 
sue. 
Lenticels occur 
_- in abundance upon 
es the tubercles, and 
are found near the 
tip of a root upon 
which young tu- 
bercles are grow- 
@ ing. They occur 
also in thickened 
primary roots 
which do not bear 
tubercles. That 
they are developed 
G. 3.— Longitudinal section of tubercle. XX 40. 5 
a, rte zone; f, tapetum-like layer, with hyphae. very early in the 


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