Ch. IV, 6] TRANSFER THROUGH PLANTS 145 



Of the two currents in the fibro-vascular bundles, the water 

 current is by far the more voluminous, because of the great 

 demands of transpiration. Some of the largest trees require 

 each day literally tons of water, which must be raised one, 

 two, or three hundred feet, and in rare cases still higher, into 

 the air. To raise a given amount of water to a given height 

 requires the expenditure of a definite amount of energy, 

 no matter whether done quietly by a tree, noisily by an 

 engine, or laboriously by human effort. It has been calcu- 

 lated that the amount of work done, and energy required, 

 to raise the water used by a large tree during twenty-four 

 hours is approximately the same as that expended by a 

 person in carrying three hundred large pailfuls of water up a 

 ten-foot flight of stairs, — that is, a pailful every two minutes 

 through a ten-hour working day. It is the botanist's prob- 

 lem to explain the source of the energy whereby such great 

 quantities of water can be raised to such heights against 

 gravitation in small tubes which entail a great deal of friction. 

 In other words, what are the forces which impel the rise of 

 the sap in trees ? 



The water in passing along ordinary stems moves chiefly 

 in the xylem part, especially the ducts, of the fibro-vascular 

 bundles. This can be proven by experiment, for if an her- 

 baceous stem be cut and stood in water dyed with some 

 obvious color, e.g. red ink, and then later, as the first traces 

 thereof appear in the younger parts, the stem be sectioned at 

 different heights, the ducts will be found filled with the red 

 fluid, which is also diffusing outward to the neighboring 

 tissues. In a tree the water runs only in the younger xylem, 

 i.e. the outer and younger rings of the white sap wood, and 

 the colored heart wood has no part in the process. Thus is 

 explained the fact that many kinds of trees can lose their 

 heart wood by decay without detriment to water conduc- 

 tion, as also the familiar fact that in tapping Maple trees 

 for their sap, it is useless to bore more than an inch or two 

 into the wood. 



