134 



PLANT ANATOMY. 



more proportional elongation. 



Bast fibers are found mainly in the 

 iuuer bark of most Dicotyledons, and are 

 distribiitod throughout the stems of 

 Monocotyledons, where they generally 

 surround the fibro-vascular bundle as a 

 protection. They, with wood fibers, are 

 specially well adapted to support those 

 plants which have to stand great strains 

 and are few or absent in plants not 

 similarly subjected, as in water plants 

 and those leaves and stems that float or 

 lie flat on the ground. (Fig. 42.) 



A variety of bast, called soft bast, will 

 be discussed under the conducting 



tissues. 

 Wood Fibers, Libriform, Woody 



Schlerenchyma, are different names giv- 

 en to the so-called woody parts of the 



plant, the greater proportion of which 

 is made up of such fibers; they are in 

 most respects similar to bast fibers, as 

 the word libriform indicates. They are 



more slender and are neither as strong 

 nor as elastic as bast fibers. In many 

 forms the pores may be circular instead 

 of slit-like. In stems of Dicotyledons 

 they occur as elements of the Xylem in- 

 side the Cambium ring, while the bast 

 fibers are found in the Phloem without. 

 In many plants, however, it is impos- 

 sible to distinguish them if isolated. 

 (Fig. 43.) In some intermediate forms 

 between wood fibers and tracheids bor- 

 der pores are to be found. 



Woody fibers contain in general noth- 

 ing but water or air, and in some cases 

 some shrunken plotoplasm remains. 



Fibrous Cells, or Intermediate Cells, 

 agree in form and in structure of the 

 walls with the woods cells, but they rep- 

 resent earlier stages, and therefore con- 

 tain more living matter than do the 

 woody fibers. Starch is nearly always 

 present in them, and sometimes chloro- 

 phyll and tannin are found. (Fig. 44.) 



CHAPTER Vni- 



TtSSUES OF NUTRITION 



1. ABSORPTION SYSTEM. This 

 consists of tissues whose function it is 

 to absorb from the earth the water con- 

 taining in solution the various mineral 

 salts that will be of benefit to the plant, 

 and which, with the food received from 

 the air, constitute its entire nourishment. 

 The part of the higher plants devoted to 

 this work is the root hair, which has 

 had a gradual development from the low- 

 est alga^ to its present structure. The 

 root hairs are true trichomes of the epi- 

 dermis of the root, and have very thin 

 walls, through which osmosis can read- 

 ily take place. (Fig. 45.) 



2. ASSIMILATION SYSTEM. The 

 main tissues of assimilation are those in 

 the leaf or leaf-like organs, and the im- 

 portant and active agents are those cells 

 that possess chlorophyll. The leaf con- 

 sists of a stalk, the blade, and, in some 

 cases, two stipules. The petiole carries 

 tlie vessels from the stem into the leaf, 

 and has various supporting elements, ac- 

 cording to the needs of the leaf. The 

 blade consists of these vessels and some 

 of their protecting elements which are 

 collectively called the ribs or veins and 

 veinlets. These ramify either in more 

 or less parallel lines, as in the Mono- 

 cotyledons, or irregularly In a reticulat- 



Fig. 45. -Yonngc roots of Triticum vulgare with 

 root hairs and attached earth. (Tschirch). 



