STEM. LATICIFEROUS TISSUES. 87 



the Elm. The sieves occur on oblique walls facing 

 the radial plane, and are therefore here seen in surface 

 view. Note that they have a similar appearance to 

 those above described, but here three or more sieve- 

 plates occur on each oblique wall. 



II. Cut tangential sections of the same ; mount as 

 before. The oblique walls are here cut longitudinally ; 

 the sieve-plates are often callous, especially in 

 autumn. 



Note the form of the segments of sieve-tubes ; it 

 is fundamentally the same as that of the cambium 

 cell, as seen in tangential section. 



LATICIFEROUS TISSUES. 



The material for the study of these tissues should be 

 prepared by treatment with alcohol to coagulate the 

 latex. Care should be taken to place the material in 

 alcohol directly it is cut, or at least the cut surfaces 

 should be wetted with alcohol so as to check the flow 

 of latex from them. If the latex be allowed to escape, the 

 laticiferous tissues are emptied, and are then much less 

 easily traced than when they are full. The best method 

 is perhaps to preserve the -whole plant without injury 

 in alcohol, in which case the latex will not be lost 

 at all. 



i. Laticiferous Vessels. 



I. Cut tangential sections from the phloem of the root 

 of the Dandelion (Leontodon Taraxacum}, mount in 

 potash and glycerine, and warm ; examine under a low 

 power. 



