28 BOTANY. 



examine a drop of the milk (latex) under the microscope by trans- 

 mitted light. When so examined it presents quite a, different ap- 

 pearance from that \>j ordinary reflected light; thus white latex 

 appears to be light granular brown. 



(a) Make thin longitudinal sections of the stem of a Milkweed 

 (Asclepias). By careful searching, tubes containing latex (appearing 

 light granular brown) may be seen. 



(J) Make a similar study of the stem of the large Spurge (Euphor- 

 bia) of the greenhouses. Its milk-tissue is thick- walled a»d easily 

 made out. 



(e) The more complex or reticulated forms of milk-tissue may be 

 obtained from the stems of wild lettuce, garden-lettuce, poppy, and 

 blood-root. 



{d) Collect a quantity of latex of a Spurge or Milkweed in a watch- 

 glass and slowly evaporate it: the residue will be found to consist of 

 a sticky, elastic material resembling india-rubber. 



47. 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-disks, and through 

 these perforations and those in the horizontal walls the 

 protoplasmic contents of the contiguous ceUs freely unite 

 (Fig. 18). 



48. 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 substance 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. 



49. 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 



