THE VEGETATIVE ORGANS OF PLANTS. 281 
the sieve-cells in the overground stem of the potato; A, 
B, cross-section of parts of vascular bundle—A, exterior 
part towards rind; B, interior portion next to pith—a, a, 
cell-tissue inclosing A 
the smaller sieve- ae 
cells, A, B, which 
contain sap turbid 
with minute gran- 
ules; 6, cambium 
cells; c, wood-cells 
(which are absent in 
the potato tuber ;) d, 
ducts intermingled 
with wood-cells. @ 
represents a section 
lengthwise of the 
sieve-ducts; and D, 
more highly magni- 
fied,exhibits the fine- 
ly perforated, trans- 
verse partitions, 
through which the 
liquid contents free- 
ly pass. 
Milk Ducts.—Be- 
sides the ducts al- 
ready described, 
there is, in many 
plants, a system of 
irregularly branched 
channels containing | 
a milky juice, as in the sweet potato, dandelion, milk- 
weed, etc. These milk-ducts, together with many other 
details of stem-structure, are imperfectly understood, and 
require no further notice in this treatise. 
Herbaceous Stems.—Annual stems of the exogenous 
