106 



BIOLOGY 



cost 



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seen four kinds of cells. There are a few large cells called sieve 

 cells, Sj and near them some small cells called the accompany- 

 ing cells, a. Other cells still smaller and with thin walls form 

 the parenchyma (Gr. para = beside + en = in + chein = to 

 pour), pa, and a few cells, with very thick walls, are called 

 the stereome cells (Gr. stereos = solid), st. The cells of the 

 cambium do most of the growing; as they multiply they pro- 

 duce new cells both on their inner and their outer edge, causing 

 the bundles to increase in thickness by additions between 

 the xylem and the phloem. 



Figure 46, a longitudinal section through a bundle, shows 

 the real shape of the cells. The cambium layer is composed 



of slightly elongated 



? ' f /? & S P f P ce ^ s w ^h sc l uare ends. 



Each of these cells con- 

 tains protoplasm and a 

 prominent nucleus, dif- 

 fering in this respect 

 from the majority of 

 the cells of the bundle, 

 which are empty arid 

 represent only the cell 

 walls from which the 

 protoplasm has been 

 removed. The xylem 

 cells of the bundle, 

 forming the wood 

 proper, show several 

 types. The large ducts 

 have peculiarly marked cell walls. Some of them show rings 

 forming thickenings on the inside of the cell wall, or the 

 thickenings may take the form of a spiral, sp. Other ducts 

 show dots or pits and various peculiar markings, d. The 

 smaller cells, the tracheids, t, are much narrower than the 

 ducts, but have relatively thicker walls. Some have square 



Phloem Xylem 



FIG. 46. LONGITUDINAL SECTION OF A 

 FIBRQVASCULAR BUNDLE 



a, accompanying cells; 



c, cambium cells; 

 co, cortex; 



d, ducts; 

 p, pith; 



s, sieve cells; 



sp, spiral ducts; 



st, sterome cells; 



t, tracheids or wood cells. 



