148 



BOTANY 



PART I 



The living elements in the wood strands always bear a distinct relation to the 

 water-courses which they accompany, enclosing them in a more or less complete 

 sheath. Tiie living cells adjoining the tracheal elements are in communication 

 with them by means of one-sided bordered pits. When such pits occur between 

 living cells and tracheal elements the pit cavities are absent on the side of the 

 living cell, but present in the tracheal elements ; they differ from the true bordered 

 pits in the absence of a torus on the pit membrane. 



The Elements of the Secondary Phloem in Gymnosperms and 

 woody Dicotyledons are sieve-tubes, or these together with companion 

 cells, bast parenchyma with abundant cell contents, and long narrow 



Fig. 155. — Transverse section of a stem of Tilia 

 ulmifolia, in the fourth year of its growth, pr, 

 Primary cortex ; c, cambium ring ; cr, bast ; pm 

 primary medullary ray ; pm' expanded extremity 

 of a primary medullary ray ; sm, secondary 

 medullary ray ; g, limit of third year's wood. 

 (X G.) 





Fia. 15(3. — A radial section of the wood of 

 Tilia uhiufolw, showing a small medul- 

 lary ray. g, Vessel ; I, wood fibres ; tin, 

 medullary ray cells in communication 

 with the water-courses by means of 

 pits ; SHI, conducting cells of the 

 medullary ray. (x 240.) 



bast fibres with strongly thickened walls. The sieve-tubes serve to 

 conduct proteid materials ; the companion cells, or in their absence 

 special rows of the bast parenchyma, take up substances from the 

 sieve-tubes ; storage and conduction of carbohydrates take place in 

 the parenchyma in which bye-products of metabolism, such as tannins 

 and calcium oxalate, also accumulate. 



As in the case of the wood, the elements of the bast may be referred to two 

 forms of tissue, tlie sieve-tube and the parenchymatous. The former is represented 

 by the sieve-tubes or by these together with comjianion cells, the parenchymatous 

 portion 1>y the phloem parenchyma and the bast fibres, between which there are 

 intermediate forms of clement. 



In the bast strands of Gymnosperms, the phloem elements pro- 

 duced by the cambium (Fig. 151 B, r) consist of sieve-tubes, the 

 parenchymatous cells of the bast parenchyma (jj and k), and in 



