36 DIFFERENTIATION OF THE TISSUES 



thin-walled cells (Fig. 16), and its chief significance here is that 

 from the division of its cells the lateral roots take their origin. 



The Primary Vascular Bundle. — The typical vascular bundle 

 of dicotyledonous stems consists, as already stated, of three parts, 

 an outer or phloem, an inner or xylem, and a median or cam- 

 bium. These, in primary bundles, are all the product of the 

 differentiation of the procambium, and the progress of their evo- 

 lution will now be followed. 



A typical phloem consists of three elements, the sieve tubes, 

 the companion- cells, and the sieve or phloem parenchyma (Fig. 14). 

 A sieve tube consists of a vertical row of cells each of which is 

 vertically elongated and separated from its neighbor above and 

 below by a thickened partition wall that is perforated by many 

 openings. These partition walls somewhat resemble a sieve and 

 have therefore suggested the name foi the tube (Fig. 14). 



In the evolution of a sieve tube a vertical row of procambium 

 cells divides longitudinally, producing a double vertical row of 

 cells. The cells of one row enlarge transversely and vertically; 

 their ti^ansverse or end walls thicken, leaving thin places or pits, 

 which finally become complete openings from one cell cavity 

 to another by absorption of the wall at the bottom of the pits, 

 and the vertical row of cells in this way becomes i continuous 

 tube (Fig. 18.) 



The vertical walls of the sieve tubes are usually con'paratively 

 thin,, but they are sometimes markedly thickened. The cells in 

 the row companion to the sieve tubes, known as the companion 

 cells, enlarge in all dimensions somewhat, but remaii" much 

 smaller than the cells of the sieve tubes. The cells of th^ sieve 

 tubes remain alive, at least throughout the first year of tht ir for- 

 mation, but they strangely lose their nuclei. This is an anomaly, 

 for cells that have been deprived of their nuclei artificially soon 

 die. It is thought in this instance that the nuclei of the com- 

 panion cells extend their influence to those of the sieve tubes 

 and so keep up there the oxidative and other processes .that 

 depend upon nuclear activity. The walls of both sieve tubes 

 and companion cells remain cellulose. 



