276 TRANSPIRATION. 



absorptive cells (beech, &c.). These bundles pass above into the leaves, forming there 

 the " veins " of the leaf-blade, which spread out into an extremely fine network of 

 tiny strands, and terminate quite close to the evaporating cells on the surface. That 

 the wood actually forms the conducting tissue of the transpiration current is satis- 

 factorily demonstrated by the existence of old trees whose trunks have long been 

 hollow, whose pith is disintegrated and fallen away, and which have also been de- 

 prived of bark around their base. In the olive plantations at Lake Garda, one of 

 which is reproduced in figure 60, many trees are to be seen in which the lower part 

 of the trunk is not only hollow and without bark, but is also often tunnelled and 

 split, so that the upper part of the tree looks as if it were raised on stilts. The only 

 communication between the soil and the upper part of the tree is by means of these 

 props, which are continuous with the roots below and are composed entirely of 

 woody cells and vessels. And yet these olive-trees are still vigorous, putting out 

 new branches and leaves every year, and blossoming and producing fruit; and 

 they derive their necessary food from the ground by supplies which have no other 

 upward path than the wood of these props. 



Moreover, by repeated experiments it has been proved that the bundles of woody 

 cells and vessels which are united together into a woody cylinder, inserted between 

 the pith and the cortex in the trunks and stems of trees and shrubs, serve as con- 

 ductors of the transpiration current. If a ring of cortex is removed from the stem 

 of a leafy plant, whose leaves are transpiring in dry air, and are supplied with 

 water from below by the transpiration current, this flow of sap to the leaves will 

 not be interrupted, and the leaves remain firm and tense. But as soon as a piece of 

 the wood is removed or the above-mentioned strands are cut through, even though 

 the cortex be left entire, the flow to the leaves stops immediately, and they become 

 flaccid and hang down in a withered condition. 



The cellular formations of the wood and strands, which function as the con- 

 ductors of the crude nutritive sap to the leaves, are as already mentioned wood- 

 cells and wood-vessels. Formerly the idea was held that these structures served for 

 the passage of air, and it was believed that they were analogous to the respiratory 

 organs the so-called tracheae of insects; therefore these wood-vessels were also 

 called "tracheae", and the wood-cells "tracheides". The wood-cells are elongated 

 chambers, on an average 1 mm. long and O'05-O'l mm. broad, and their walls are 

 unequally thickened, either by reticulate or annular bands, or spiral threads project- 

 ing slightly from the inner wall into the lumen, or by so-called bordered pits, which 

 are represented in fig. 10 1 and fig. 10 2 . The wood-vessels are tubular, and very long 

 in proportion to their width, which is never more than a fraction of a millimetre; 

 they extend uninterruptedly through stalks, branches, leaves, perhaps even through 

 the entire plant from the root-tip to the crown. They are composed of rows of cells 

 whose separation walls have been broken down. The walls of the wood-vessels 

 exhibit similar thickenings to those of the wood-cells or tracheides. When the 

 chambers and tubes of the wood, with their bordered pits and projecting bands, are 

 fully developed, the living protoplasm which carried on the building forsakes the 



