106 TRANSPORT OF WATER AND SOIL SOLUTES 



descending into the stem to take on and conduct to it directly the 

 water and solutes which it as a transpiring and photosynthesizing 

 member demands. 



The Tracheids. The tracheids are elongated cells espe- 

 cially adapted to be water carriers by numerous thin places 

 in the walls in the form of bordered pits or associated with spiral, 

 annular, or reticulate thickenings (Fig. 19). Each tracheid is a 

 single closed cell, and water and solutes flowing into and out of 

 it find the thin places suited to their passage. 



The length of the tracheids varies a good deal; from i to 

 2 mm. is a common length, but they are often shorter, 

 and they may become many times longer than this. It there- 

 fore appears that a water-conducting system composed of tra- 

 cheids would offer more resistance to the flow of water than 

 one made up chiefly of tracheal tubes, because of the less fre- 

 quent cross walls in the latter case. 



Tracheids commonly occur associated with the tracheal tubes, 

 but they are sometimes lacking. They usually replace the 

 tracheal tubes in the smaller ramifications of the veins of leaves, 

 and where tracheal tubes anastomose, as notably at the nodes 

 of grasses, the connections are commonly made by tracheids; 

 and frequently tracheal tubes in successive annual rings of growth 

 communicate by means of tracheids. 



Comparing the use of tracheal tubes and tracheids among 

 the Monocotyledons and Dicotyledons it appears that the tra- 

 cheal tubes are most in use for conduction through long dis- 

 tances, while for transport through short distances the tracheids 

 are preferred. 



In Pteridophytes tracheids occur more frequently than tra- 

 cheal tubes, and in Conifers tracheids constitute the sole water- 

 conducting tissue, excepting special water-conducting paren- 

 chyma in the medullary rays; and the nearest approach to tra- 

 cheal tubes is found in the elongated, spirally-thickened tracheids, 

 which as a product of the procambium alone, occur next the pith. 



The wood of the pine, save the medullary rays, and a small 

 amount of wood parenchyma devoted to the secretion of resin, 



