PLANT WORLD CONTINUED 



235 



nified tissue is arranged in the form of a continuous spiral-band, or col- 

 lection of bands). 



Tracheids are merely single cells which have lost their protoplasmic 

 contents, but not their entire end-walls. Communication is carried on 

 by pores in the vessel walls. 



Sieve Tubes, unlike all the other ducts mentioned above, usually 

 carry materials away from the leaves. They are merely individual cells 

 whose end-walls have not completely 

 broken down, as in the tracheids, but 1 



have formed sieve plates with many 

 pores or perforations connecting one 

 such individual cell with the next be- 

 low, and so continuing for great 

 lengths in the plant. The walls of 



Fig. 137. Mechanical and Supporting Tissues. 



These tissues consist of wood and bast fi-bers (See Fig. 130), schlercnchyma 

 (stone-cells), and collenchyma. 



A, Portion of epidermis and collenchyma from the stem of Rumex crispus. 

 Cross t< ction, ep, epidermis ; c, collenchyma. 



B, Sclerotic cells from the root of Apocynum androsaemifolium. All highly 

 magnified. (From Bastin's "College Botany" courtesy of G. P. Engelhard & Co.) 



C, 1. Peppermint stem. Arrangement of collenchymatic (C), tissues at angles 

 of the stem. 2. Peppermint stem. 3. Sabal seed. 4. Colchicum seed. (Porous 

 type). 5. Nux Vomica seed. (Striated type). 6. Arrangement of collenchymatic 

 tissues around the midvein of a leaf. C, collenchyma. (From C. W. Ballard's 

 "Vegetable Histology," Courtesy of John Wiley & Sons). 



sieve tubes are composed of cellulose, there being no trace of lignifica- 

 tion. 



Medullary Rays furnish the method by which material is transported 

 from the inner-tube region of the plant, laterally, to the tissues which 

 lie closer to the outside of the stem, and from these to the pith where 

 food may be stored. 



Latex Tubes. These are non-porous tubes in certain plants and 

 contain a milk-like fluid. 



. Porous Parenchyma. In the pith region the parenchyma, which is 

 very porous, may possibly assist in permitting the nutrients which are 

 in solution to pass back and forth. 



