198 Atkinson . — A Monograph of the 
while peripheral growth forming the tie-cells proceeds more 
slowly and the cells are shorter and broader. Terminal and 
peripheral growth now proceed in like manner, the termina- 
tions of the ascending and descending generative filaments of 
adjacent segments of the sexual shoot remaining in contact ; 
the tie-cells produce peripherally two to three cells of the 
medullary layer ; these in turn produce in the same manner 
three to several cells of the intermediate layer, and these in 
like manner three to several cells of the cortex. While the 
cells of the medullary layer are forming they increase rapidly 
in size, and the edges of the corymbose branching of the four 
rays remain in contact a short distance from the axis, and a 
hollow tube is the result. 
In detail the process varies for each subgenus and is as 
follows. The centrifugal growth of the primary cell proceeds 
for a time, when fission takes place and five daughter-cells 
result. The line of fission proceeds in such a manner as to 
give four concave faces to the peripheral contour of the 
central axis, and to make the four peripheral cells of unequal 
size (Fig. 30, cross section). Two opposite cells ( a , a) are 
larger than the other two opposite cells (b, b'). The outline 
of the four cells is somewhat trapezoidal, the larger cells have 
the broader end distal, and the smaller ones, proximal. Each 
of these cells is a mother-cell of a ray-cell and of one or two 
ascending and descending generative filaments. 
Subgenus Sacheria . The two larger cells each produce one 
descending and two ascending generative filaments, and one 
ray-cell. Usually the first line of fission in any one of the 
cells is transverse and starts about two-fifths the way from 
the distal end on the outer surface and runs obliquely distal 
toward the central axis. After growth proceeds for a short 
time, the second line of fission, usually transverse, starts about 
the middle of the proximal cell on the outer surface and runs 
obliquely proximal toward the central axis. Sometimes this 
last line of fission is preceded by longitudinal fission of the 
first cell, so that the two ascending generative filaments are 
marked off before the descending one is differentiated from 
