20 BOTANT. 



position it is quite certain that the tuhes 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. (fi) The other form is that composed of reticfulately 

 anastomosing vessels. Here 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, J. 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 

 (Pig. 13). 



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 thp 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 latticed 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 



