142 ELEMENTS OF PLANT ANATOMY. 
in which the parenchymatic and medullary ray cells are still 
living. 
ABNORMAL CASES OF SECONDARY GROWTH. 
Secondary growth occurs so seldom among the monocotyle- 
dons that the few cases of it may be classed with the abnormal 
cases of the dicotyledons. Among the latter class there are 
many variations from the type described; a few prominent ones 
may be mentioned here. In certain genera, the cambium of the 
ring loses power to grow after a time and another cambium ring 
originates outside it. In Cocculus laurifolius and some others 
of the Menispermaceae this new ring forms in the primary rind; 
in Phytolacca, Wisteria and some species of Bauhinea, in the 
phloem tissue of the primary bundles. This new ring develops 
phloem outside and xylem inside just as the old one did, and 
may in its turn be replaced by a succession of other rings. 
Some of the Cycads form outer rings in a similar manner, 
making a complcated structure. Tecoma radicans develops a 
cambium ring on the inside next the pith, which cuts off new 
cells in contrary directions from those of the first or normal 
cambium ring, that is, phloem inward and xylem outward. In 
Mirabilis Jalapa the leaf-trace strands form a somewhat irregular 
collection deep in the pith, while the cambium ring is formed 
from stem bundles. m 
Rhizomes of certain species of Rheum have a singular 
structure, the leaf-trace bundles forming a normal ring which 
develops regularly, enclosing a relatively large pith. Through 
this pith, in the early stage, run other vascular bundles obliquely, 
anastomosing with each other and with the leaf-trace bundles. 
At first these consist simply of sieve-tubes and their accompany- 
ing cells; later they surround themselves each with a cambium 
ring which develops phloem inside and xylem outside. 
Still more remarkable forms are seen in the climbing stems 
of Sapindaceae. Sarjania is the one best known. Here ona 
