438 BOTANICAL GAZETTE [JUNE 
ligno-cellulose in distinct layers, and these are pierced by numerous 
simple and branched pits. 
Primary medullary rays. 
The primary medullary rays extend from the cambium to the 
outer cortex, the ray cells merging into the cortical cells without a 
distinct line of demarcation. The outer portion of the ray is of 
course directly differentiated from the meristem at the growing point, 
while the inner portions owe their existence to the activity of the 
cambium. Some of the cells in the outer portion retain their power 
of growth and division for several years, the majority of the dividing 
i any. 
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Fic. 5.—Cross section through old bast. X25. Fic. 6.—Radial section through 
old bast. X75. c, cork; cam, cambium ring; e, epidermis; m, medullary ray, 
p, parenchyma; ph, secondary phloem; phel, phelloderm; sc2, secondary sclerenchyma; 
St, stone cells; x, xyiem. 
walls being radial. Thus, in a stem one year old, the strands of 
primary sclerenchyma are separated by from two to five layers of cells, 
in a stem two years old by as many as fifteen layers, while in a stem four 
years old the number may be increased to thirty or more. Since the 
portions of the rays derived from the cambium do not undergo further 
divisions, they remain permanently from one to three cells in width. 
In consequence of these facts the rays gradually assume a T-shape 
a 
