20 BOTANY. 
position it is quite certain that the tubes of this form of 
milk-tissue frequently replace bast-fibres. In other cases, 
however, they appear not to be of this nature, but to arise 
from the soft tissue by the absorption of the horizontal 
partition-walls. 
44, (2) The other form is that composed of reticulately 
anastomosing vessels. Tere the tissue is the result of the 
fusion of great numbers of short cells. The walls are thin 
and often irregular in outline. In chicory, lettuce, etc., 
this form of milk-tissue is very perfectly developed as a 
constituent part of the outer portion of the woody bundles 
(Fig. 12, A and B). 
45. Sieve Tissue.—As found in the flowering plants this 
tissue is for the most part made up of sieve-ducts and the 
so-called latticed cells. The former (the sieve-ducts) con- 
sist of soft, not lignified, colorless tubes of rather wide 
diameter, having at long intervals horizontal or obliquely 
placed perforated septa. The lateral walls are also per- 
forated in restricted areas, called sieve-discs, and through 
these perforations and those in the horizontal walls the 
protoplasmic contents of the contiguous cells freely unite 
(Fig. 18). 
46. The tissue composed of these ducts is generally loose, 
and more or less intermingled with soft tissue; in some 
cases even single ducts run longitudinally through the sub- 
stance of other tissues. In the form described above it is 
found only as one of the components of the outer or bark 
portion of the woody bundles of plants. 
47. The so-called Jatticed cells are probably to. be re- 
garded as undeveloped sieve-ducts, and hence the tissue 
they form may be included under sieve-tissue. Latticed 
cells are thin-walled and elongated; they differ from true 
