STRUCTURE AND PHYSIOLOGY OP THE PLANT 



1 



Fig. 17. 

 Begonia floweis, 

 showing tbe 

 sexes separate. 

 Stamlnate or male flower 

 above; pistillate or female 

 beneath. The seed-pod or 

 ovary is shown at B; at A 

 there is none. 



end to end or closely interlapping, are usually associated in more or less deiinite strands or bundles 

 (Fig. 21). It is these strands, or parts of them, that produce the commercial fibers. 



The bundle, running lengthwise the stem, is composed of two parts or regions : the xylem or wood 

 part, lying on the inward side of the bundle as it stands in the stem ; the 

 phloem or bark part, lying on the outward side. These bundles stand side by 

 side around the outside of the woody cylinder, with the pith or undifferen- 

 tiated parenchyma at the center of the cylinder. These bundles therefore 

 make a continuous ring. However, the bundles are themselves supplied, 

 when growing, with living parenchyma, called cambium, from which new cells 

 are formed for both the xylem and phloem regions of the vascular bundles. 

 Inasmuch as the bundles form a ring about the stem, so the cambium that 

 accompanies them also forms a ring. The parenchyma tissue extends outward 

 from the pith between the bundles (or the bundles are 

 imbedded in the parenchyma), causing the rayed appear- 

 ance of the stem in cross-section. 



The xylem part of the bundle contains the trachear 

 like spiraled or pitted vessels. These are the routes 

 through which the water ascends from the root. The 

 phloem part containing the sieve-tubes transports the 

 organized food, or "elaborated sap," after it has been 

 formed in the leaves ; this food is transported to all parts 

 of the plant to build new cells, or sometimes to be stored 

 until needed. The supporting tissue may be associated with 

 the vascular, or fibro-vascular, bundles. Bast is schleren- 



chyma tissue growing with the phloem. The xylem and phloem regions separate 

 as they grow, the former becoming part of the wood and the latter part of the 

 inner bark. The outer separable part commonly called bark is a very complex 

 structure, being formed of thp cortex or skin of the stem and the cork and 

 strengthening tissues formed therein, the old and dead or dying phloem, and the 

 new phloem that is just forming from the cambium in the vascular bundle. The 

 xylem grows old and dies ; the dead tissue becomes filled and hardened in firm 

 wood ; new xylem tissues are 

 added on the outward side. The 

 phloem grows old and dies ; the 

 dead parts are added to the bark ; 

 new phloem tissues are added on 

 the inward side. The fibers of hemp 

 and flax are derived from the 

 phloem. 



In monocotyledonous plants, as 

 grasses, sedges, orchids, bananas, 

 palms and all lily-like plants, there 

 are vascular bundles with xylem 



each bearing one long and phloem regions, but the bun- 

 petal or ray. ^jgg ^^^ scattered through 



stem and therefore do not form an exterior ring, 



there is no true pith. Moreover, these bundles do not 



contain cambium, and therefore, the stem does not 



increase much in thickness and does not have a distinct 



separating bark (Pig. 22). The fibro-vascular bundles are 



very evident in the stem of Indian com, and can be pulled 



out. There are some commercial fibers produced by 



plants of the dicotyledonous kind. Manila hemp is from a species of banana, and sisal hemp, 



from an agave, one of the century plant group ; these fibers are derived from the entire bundle, 



both xylem and phloem, and this origin probably accounts for their stiffness and hardness and their 



resistance to abrasion. 



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Fig. 16. A daisy or white- 

 weed, one of the com- 

 • positse. Very many 

 flowers compose the 

 head, the outer ones 



1 the ^-^^llH/ 







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m 



Fig. 18. Separated sexes in black walnut. The stam- 

 lnate flowers (in clusters called catkins) atBj pistil- 

 late flowers, each with two stigmas, at A. 



