62 METABOLISM 



on the wood as an immense water reservoir, which in times of abundance (e. g. 

 when transpiration is retarded) is full, and in times of scarcity (e. g. under 

 continuous drought) is depleted. For this reason it is impossible to draw 

 any reliable conclusions as to the amount of water carried by the stem from 

 a calculation of the amount of transpiration taking place during a single day. 



The amount of transpiration taking place in a tree can be estimated only 

 with an approximation to accuracy, and even though we knew the number and 

 average diameter of the vessels we would still be ignorant as to how many of 

 them took part in the transport of water. The fact is that the whole of the 

 wood in the transverse section of a tree behaves by no means uniformly in this 

 respect. To begin with, all the so-called heart wood may be neglected, because 

 the lumina of the vessels in this region are quite unable to conduct water owing 

 to thyloses and other obstructions. In typical heart wood trees, such as, for 

 example, the oak, an incision, sufficiently deep to interrupt the continuity of 

 the sap wood, is enough to arrest the conduction of water. Indeed in many 

 cases this layer of sap wood is so thin that mere ringing of the bark impairs, 

 and even interrupts, the supply of water (Rhus typhina}. On the other hand, 

 there are trees which form scarcely any heart wood and whose older wood 

 retains the power of conduction. To this class the lime belongs, and it will 

 be remembered how it was shown in the last lecture that this plant is capable 

 of withstanding for many years the injurious effects of ringing. In an ordinary 

 heart wood tree, on the other hand, the sap wood beneath the place where the 

 operation of ringing has been performed is rapidly destroyed and the apex 

 withers. If a sharp line of demarcation exists between the heart and 

 sap wood, and if we assume, on the one hand, that the whole of the sap 

 wood is of equal value for conducting purposes and that the whole of the 

 heart wood is valueless, still only approximate estimates of the area of the part 

 of the wood actually concerned in the transport of water can be given. The 

 transition between heart and sap wood is, however, quite gradual, and not in- 

 frequently the lumina of the vessels formed in the second and third years of 

 growth are blocked up by thyloses (Robinia, WIELER, 1888). Generally 

 speaking, the youngest annual ring is the most effective water conduit, and the 

 capacity for conducting water gradually decreases as the centre is approached, 

 where possibly the tissue plays the part of a water reservoir only. 



An estimate of the rapidity of movement of water based on the transverse 

 area of the conducting system and on the amount of water flowing through 

 it is, likewise, out of the question. A rough idea of the amount, however, 

 may be arrived at by employing the method perfected by SACHS (1878), but 

 based on the experiments of McNAB (1871) and PFITZER (1877). SACHS allowed 

 the plant to absorb a solution of lithium nitrate through the root, after finding 

 that this substance penetrated the protoplasm very rapidly without doing it any 

 injury and passed into the vessels as quickly as did the water itself. Since 

 lithium does not as a rule occur in the plant, and further, since the minutest 

 trace of that substance can readily be detected by the spectroscope, the rate 

 of its ascent could be easily determined. Some of the data with regard to the 

 hourly ascents which SACHS obtained are summarized in the following table : 



Acacia lophantha ..... (aver.) 154-0 cm. 



Nicotiana tabacum 118-0 



Musa sapientum (aver.) 100-0 



Cucurbita pepo 63-0 



Podocarpus macrophylla . . . . . , 18-7 ,, 



It must not be supposed that these numbers indicate either the highest 

 or the lowest extremes of rapidity with which the fluid may ascend the tree. 



The height to which water must in the long run be raised is, in the case of 

 some plants, very remarkable. Eucalyptus amygdalina, with a height of 



