68 



METABOLISM 



tance ; thick and thin regions always alternate. The thickenings are annular, 

 spiral, or reticulate. In the spiral and annular vessels the thin portions are 

 of similar form to the thick parts and alternate regularly with them ; in the 

 reticulate type, however, the thin places are circular or elliptical and known 

 by the name of ' pits '. It may be logical, but scarcely customary, to call the 

 thin places in the first two cases ' pits ' also, all the more as there is no sharp 

 line of demarcation between the three types. This is not the place to enter 

 into a discussion of their other peculiarities, though an appearance of very 

 general occurrence may be noted, viz. the attachment of the thickened regions 

 to the original unthickened wall by a somewhat narrower base (Fig. 17), so 

 that a transverse section of the thickening presents a |- -shaped appearance. 

 This structure in ' annular ' and ' spiral ' vessels was for long overlooked, and 

 was first demonstrated by ROTHERT'S (1899) elaborate researches. 



One form of reticulate thickening, on the other hand, has been often in- 

 vestigated and known by the name of ' bordered pits '. Looked at on surface 

 view (Fig. 18, 2) the circular or elliptical mouth of the pit appears surrounded 

 by two lines enclosing a ring-like area. It is easily understood that the cavity 

 is due to the deposit of thickening material on the outer rim of the pit, the 

 widening of which brings about the formation of the cavity ; pit canals of 



Fig. 17. i. Spiral vessel of Cucurbita 

 in longitudinal section. (After Ro- 

 THKRT, 1899, pi. 6. fig. II. x AOO.) 

 2. Reticulate vessel of Opuntia. (After 

 ROTHERT, 1899, pi. 6, fig. 30. X 400.) 



Fig. 18. /. Bordered pit ofPinus 

 sylvestris in tangential section ; /, 

 the torus, z. Surface view of the 

 pit ; corresponding regions are 

 united by dotted lines j. As in /, 

 but the torus is pressed against the 

 pit on the left. (After Russow, 

 diagrammatic.) 



equal width throughout have no such cavity. Bordered pits occur especially 

 frequently on walls which are common to two vessels and are then (as in Fig. 18) 

 bilaterally symmetrical. The median closing membrane possesses the further 

 special peculiarity of having in the middle a lens-shaped swelling (torus) sharply 

 deliminated from the very thin remainder. The closing membrane is not always 

 found medianly situated, as is shown in Fig. 18, / ; it is capable of moving in the 

 pit cavity, and, in extreme cases, may be pressed up against one or other of 

 the exits of the cavity (Fig. 18, j). 



The structure of the bordered pits, as well as the wall of the vessel in general, 

 as is easily seen, must be conducive to the transference of water into a vessel 

 from any element in the vicinity (parenchyma cell or vessel). The walls of a 

 vessel, saturated with water, like those of an ordinary cell, are swollen, and they 

 also allow water to pass through them, although they obviously present a certain 

 amount of resistance to the passage, a resistance which must, caeteris paribus, 

 be all the greater the thicker the wall. The closing membranes of the bordered 

 pits will also easily permit the passage of water, but the thickened regions will 

 present resistance. Experimental research amply confirms this a priori thesis. 

 Such researches are most conveniently carried out on Coniferae, because in these 

 plants the wood consists entirely of tracheids arranged with great regularity. 



