156 



BOTANY 



PART I 



cambium. This produces little or no xylem, the new elements being 

 mostly added to the phloem. Any elements formed on the side next 

 the xylem are water-conducting, while those adjoining the phloem 



mostly become sieve-tubes. 





wm^m 



cr^ 



f 





-f 



^W^M^^p 



Pio. 164.— Transverse section of the stem of Cordyline 

 (Dracaena) rubra, f, Primary vascular bundles ; 

 /', secondary vascular bundles : f", leaf - trace 

 bundle within the primary cortex ; ?/!, parenchy- 

 matous fundamental tissue ; s, bundle-sheatli ; t, 

 tracheides ; c, cambium ring ; cr, cortex, the outer 

 portion being primary, the inner secondary cortex ; 

 ph, cork cambium ; I, cork ; r, bundles of raphides. 

 (X 30.) 



The leaves of Coniferae live, 

 according to S. Simon, about 

 seven years, those of other 

 evergreen plants usually no 

 longer than two seasons (^**''). 

 Periderm Q-^"'). — It is very 

 seldom that the epidermis, 

 by the division of its own 

 cells, is in a condition to keep 

 pace for any length of time 

 with the increasing dimensions 

 of the stem. This, however. 



Fio. 165. — Diagram of the secondary 

 growth of tlie stem and root of 

 Dracaena, pc, Central cylinder ; s, 

 secondary tissue ; e, cambium in- 

 dicated by a dotted line ; w, root. 

 (Adapted from Schoute.) 



is the case in the Mistletoe {Fiscum album), the number of epidermal 

 cells of which is continually augmented by the formation of new 

 lateral walls, and new cuticular layers arc formed where the old 

 cuticle becomes ruptured. On older portions of the stem, however, 

 thickening layers form in single epidermal cells or groups of 



