AMOUNT OF WATER TRANSPIRED. 



229 



interest to know how high the possible maximum of the water carried up by the 

 current may rise. That a tolerably vigorous Tobacco plant at the time of flowering, 

 or a Sunflower of the height of a man, or a Gourd-plant with fifteen or twenty large 

 leaves, takes up and transpires 800-1000 cubic centimetres of water during a warm 

 July day, is certainly no rarity ; and so far as may be judged from the quantities 

 made use of by branches placed with the cut surface in the water, it may be 

 believed that large fruit-trees, or Oaks, Poplars, &c., absorb, transport through the 

 stem, and transpire from the leaves 50-100 and more litres of water daily ^ Now, 

 these large masses of water are carried up, in tall trees, to a height of 50-100 metres 

 in a direction opposed to that of gravity ; and it is clear that in these cases the plant 

 performs a work, the magnitude of which is evident most simply by supposing the 

 same mass of water to be drawn 

 up to the height stated by means 

 of a windlass, for instance, 

 worked by a man. The plant 

 of course accomplishes this 

 work in quite another way, and 

 we will now see how it does it. 

 In the first place it is 

 important to establish in what 

 part, i. e. in which tissue of 

 the root, stem, and leaves the 

 water moves. The answer, as 

 already known for nearly 200 

 years, may shortly be given 

 thus : in the proper woody 

 plants (i.e. the Conifers and 

 Dicotyledons) the ascending 

 stream of water passes through 

 the wood. In the case of the 

 plants mentioned the proof 

 of this assertion is easily fur- 

 nished ; it is simply necessary 

 to separate and remove a ring 

 of cortex, by means of a double 



annular cut on a stem, to completely interrupt' all the layers of tissue lying outside 

 the mass of wood. It is advisable to wrap tinfoil, or some other body which 

 prevents drying up, closely round the exposed wood. If the ascending current of 



Fig. 192.— Transverse section of the wood of Rhamnus Fraiigiila. g autumn- 

 wood of the older, w vessels of the spring-wood of the younger, annual ring 

 (afier Rossinann). 



' Although it is not possible to state for a particular plant any definite quantity of water which 

 it must transpire in order to flourish vigorously (and it is certain that every land-plant has wide 

 limits in this respect), it is nevertheless of value for general guidance to know approximately, on the 

 base of experimental determinations, how large the quantity of water transpired by a plant during its 

 whole vegetative period may be imder certain circumstances. According to Haberlandt (cited by 

 Pfeffer, ' Pflanzen-physiologic,^ B. I. p. 153) the water transpired by the Maize in the course of a 

 vegetative period of 173 days = 14 litres ; by the Hemp = 27 litres in 140 days; by a Sunflower = 

 66 litres in 140 days. According to Höhnel, a hectare of beech forest, 115 years old, evaporates 

 2'3 to 3'5 millions of litres of water between June i and December i. 



