THE PLANT-CELL 



59 



Conductive Tissues 



Besides the tracheary tissue already referred to, there are other 

 forms of conducting tissue met with. Most important are the sieve- 

 tubes (Fig. 40) found in the outer or bast portion 

 (phloem) of the vascular bundles. The sieve- 

 tubes closely resemble the tracheae of the woody 

 part of the bundle, but differ in not having the 

 walls lignified, and in retaining the living cell- 

 contents. While the tracheae are mainly con- 

 cerned with the conduction of water, the 

 sieve-tubes are the important agents in the 

 transfer of assimilated food-elements. Very 

 similar in appearance to the sieve-tubes of the 

 vascular plants are those found in many of 

 the large Kelps, or Brown Algae. 



Another type of conducting tissue is seen in 

 the so-called Laticiferous ducts, which occur 

 in plants with milky juice, like the Poppy, Let- 

 tuce, Milkweed, Euphorbia, etc. Sometimes FIG. 40. Longitudinal 

 the latex is red, e.g. Bloodroot (Sanguinaria), 

 yellow (Algernons), or colorless (Eschscholtzia). 



The laticiferous 

 ducts may be 

 either very long 

 and branched 



single elements, e.g. Euphorbia, or 

 the much more common irregularly 

 branching system formed by the 

 coalescence of many cells (Fig. 41). 

 It is somewhat questionable how far 

 the laticiferous ducts are of impor- 

 tance in the transfer of plastic mate- 

 rials. Much of the contents are 

 apparently excretions, whose func- 

 tions, if any, are not certainly known. 



Special Secretory Cells 



Special secretory cells are of wide 

 FIG. 41. Anastomosing laticiferous occurrence. Such are the cells secret- 

 vessels from the stem of Sonchus { th various aromatic substances 

 oleraceus. 



to which plants owe their character- 

 istic odors. The oil-glands in the Orange and Lemon belong to this 

 category, as do the mucilage and oil-cells in many Liverworts. 



section of part of a 

 sieve-tube of Megar- 

 rhiza Californica ; 

 the cytoplasm has 

 heen contracted by 

 the action of alcohol 

 (X600). 



