68 



BOTANY 



PART I 



material, so that these different substances follow nearly the same 

 course though usually in opposite directions. This tissue system may 

 in its origin be primary or secondary. 



Such complete vascular bundles contrast with the less dense 

 surrounding tissue by the narrowness of their elements, and the absence 

 of intercellular spaces ; they are often visible to the naked eye as in 

 the translucent stems of Impatiens parviflora. Strands of tissue of two 

 sorts are to be distinguished in each bundle, 

 the vascular portion or XYLEM, and the sieve- 

 tube portion or PHLOEM. The xylem and 

 phloem may be variously arranged in the bundles, 

 the transverse sections of which differ accord- 

 ingly (cf. p. 99). 



Other names are used in the literature for the complete 

 conducting bundle and its parts. Thus the conducting 

 bundles are also termed vascular bundles, fibro- vascular 

 bundles, or mestome ; the xylem is spoken of as the 

 woody portion, vascular portion, or hadrome ; and the. 

 phloem as bast or leptome. 



5. Secretory Cells and Secretory Tissue. 

 (1) SOLITARY CELLS. Secretory cells isolated or 

 arranged in rows are of frequent occurrence in 

 the most diverse tissues. They may be isodia- 

 metric or tubular, and contrast with the other 

 cells by reason mainly of their contents. Within 

 their diminished or dead protoplasts secretions 

 of the most varied kinds are contained. These 

 are end products of the metabolism and may have 

 an ecological significance as protective substances. 

 Mucilage, gums, ethereal oils, resin, gum-resin, 

 FIO. 72. Portion of a latici- tannin, alkaloids, and crystals of oxalic acid 

 ferous cell of Ceropegia. / Fi 22) are among the most frequent secretions. 



(x 150. After STRAS- \.. & ' , ,, . 



BURGER.) The walls of these cells are often subensed. 



The non-septate LATICIFEROUS CELLS which 



contain the secretion called LATEX belong here. They are richly- 

 branched tubes without cross walls, with a smooth elastic cellulose 

 wall that is usually unthickened (Fig. 72). They have a layer of living 

 protoplasm with numerous nuclei lining the wall and sometimes 

 contain starch grains ( 47 ), which in many Euphorbiaceae are dumb- 

 bell-shaped. Their cell sap is a milky, usually white, watery fluid 

 which rapidly coagulates on exposure to the air. 



Enzymes (inFicus Carica and Carica Papaya peptonising enzymes in addition), 

 tannins, poisonous alkaloids, and especially calcium -malate, occur dissolved in 

 the latex. As droplets in an emulsion gum -resins (mixtures of gum and 



