Ch. IV, 6] TRANSFER THROUGH PLANTS 



147 



passage of water moving at the ordinary rate, while resisting 

 any forcible rush of the water along the stem under suddenly 

 developed pressures. Thus the tracheids and ducts form 

 water-conducting systems of unlimited length, even though 

 the length of the individual elements is restricted. These 

 thin areas, however, exist not only at the ends of the ducts, 

 but throughout their 

 lengths, where some- 

 times they appear as 

 bordered pits in an 

 otherwise thickened 

 wall, as is very charac- 

 teristic of the conifer- 

 ous wood (Fig. 104) : 

 or else as the meshes 

 of a reticulation : or 

 as thin parts between 

 spiral or annular thick- 

 enings (Fig. 101), — all 

 of which distinctive 

 arrangements represent 

 different ways of com- 

 bining a thickening of 

 the walls with the 

 presence of thin places through which water may move to 

 other ducts or tracheids, or to neighboring tissues. The 

 annular and spiral markings are usually found in ducts or 

 tracheids of the primary growth, in which elongation is still 

 in progress, while other kinds occur in the secondary growth, 

 where elongation has ceased. Both tracheids and ducts, 

 when mature, are without protoplasm, forming non-living 

 tubes. Their mechanism, as a water-conducting system, is 

 shown in our diagrammatic figure 105. 



The all-important question as to the forces by which the 

 Water is lifted through the ducts has been answered by in- 

 vestigators in several different ways. In earlier times it 



Fig. 102. — Cross section through the 

 wood of Pine ; highly magnified. 



The cells are mostly tracheids with bor- 

 dered pits, visible in the walls. Note the 

 medullary rays and the abrupt transition 

 from autumn to spring growth. In the 

 autumn wood is a resin canal. The line 

 in all of the walls is the middle lamella, i.e. 

 a plate representing the wall first formed 

 before thickened by additional layers, and 

 somewhat different in chemical and physi- 

 cal composition from the latter. (Reduced 

 from Cavers, Practical Botany.) 



