468 A TEXTBOOK OF THEORETICAL BOTANY 



which it Hes. It is much narrower than the sieve tube and has a nucleus 



and abundant cytoplasm. The wall 

 between it and the sieve tube has 

 numerous simple pits but no pores. 



J dpi 



cp 











B 



m 



Fig. 455. — Comparison of sieve tube 

 elements. A, Juglam nigra, with 

 multiple sieve plates on oblique 

 end walls. B, Rohinia pseudacacia, 

 with simple transverse sieve plates. 

 Both types also show lateral sieve 

 fields. {After Eames and McDaniel.) 



Phloem Parenchyma 



Like the vessels, the sieve tubes are 

 constantly associated with thin-walled 

 parenchyma cells, which, unlike the 

 companion cells, are often filled with 

 starch. This is called the phloem 

 parenchyma. The tissue formed of 

 sieve tubes, companion cells and 

 phloem parenchyma is called col- 

 lectively the phloem, which together 

 with the lignified xylem form the 

 vascular tissue. 



Cambium 



Lastly we must mention a type 

 of meristematic tissue which has the 

 special function of producing the vas- 

 cular elements, both lignified and non- 

 lignified. This is the cambium. It 

 is a direct downward continuation of 

 the apical meristem, but its cells have 

 a peculiar, prosenchymatous shape. In 

 transverse section they are narrowly 

 rectangular, the longer diameter of the 

 rectangle always lying tangentially to 

 the axis of the plant. Vertically they 

 are chisel-ended, like the tracheids, the 

 points alternating to the right and left 

 sides (Fig. 456). Like other meristem- 

 atic cells they have thin walls, the radial 

 walls being the thickest. 



Epidermal Outgrowths 



Epidermal cells give rise to a variety of specialized outgrowths grouped 

 under the general name of trichomes, which includes all kinds of hairs and 

 structures derived from hairs (Fig. 457). 



The simplest kind of hair is a unicellular, tubular outgrowth from a 

 single epidermal cell. If it is very short it may be no more than a papilla. 



