THE CONDUCTION OF WATER. I 59 



incrustation of a saline nature which has the power of absorbing hygroscopic 

 water (MARLOTH, 1887). Hairs which give off acid secretions have frequently 

 been described, e. g. by STAHL (1888), in Cicer arietinum, Circaea lutetiana, and 

 Epilobium hirsutum, while they are widely distributed in insectivorous plants, 

 where, in addition to an acid, a proteolytic enzyme is also present (compare 

 Lecture XV). In many Fungi also, both unicellular types, such as Pilobolus, 

 and multicellular (Penicillium, Peziza sclerotiorum, Merulius lachrymans, Clavi- 

 ceps purpurea, &c.), drops of fluid have been observed to be excreted, which on 

 analysis are often found to be rich in organic materials, such as oxalic acid and 

 sugars of various kinds. The excretion of sugar is a frequent phenomenon in 

 the higher plants in the nectaries which, although they occur more especially in 

 the floral organs, are found on vegetative organs as well. There is practically 

 no difference in fundamental character between the water glands we have hitherto 

 been discussing and these nectariferous cells, so that, beyond referring to the 

 literature on the subject (WILSON, 1881, PFEFFER, 1892, BUSGEN, 1891, HAUPT, 

 1902) we need not go further into the question, more especially as it would 

 lead us too far away from our present subject. 



The general survey of all the phenomena, which we may group together 

 under the head of exudation of water, shows us how varied these processes are, 

 looked at from a purely physiological point of view ; it can scarcely be 

 expected, therefore, that they will all fulfil the same or even similar functions 

 in the plant. The significance of nectaries is best known ; they induce insects 

 to visit the plant, and thus in very many cases facilitate the transport of pollen 

 to the stigma. Quite as familiar is the significance of the secretion which 

 occurs in insectivorous plants, a secretion which we shall have to study in 

 detail later on. In this case the secretion aids in the digestion of captured 

 insects, and in most cases indeed it is produced only when the opportunity 

 for such digestion occurs. 



It is not so easy to ascribe biological meanings to some of the other secre- 

 tions mentioned in the preceding pages. If, along with the water, large amounts 

 of common salt or of carbonate of lime are given off, we may assume that the 

 plant is in this way ridding itself of superfluous or injurious substances. The 

 lime, it is true, may be found in many plants deposited within the body either 

 as oxalate in the cell-cavity, or as carbonate in the cell-wall ; but it would 

 scarcely be correct to conclude that when it is excreted to the exterior its actual 

 excretion serves any special purpose, the more so as silicic acid does not lend 

 itself to such excretion. Common salt, on the other hand, as we shall have 

 occasion to see later on, may cause direct injury to the plant, and since, in its 

 case, the precipitation of the fundamentally potent element chlorine cannot 

 be effected by the formation of an insoluble compound, there is no difficulty 

 in apprehending the advantage the plant obtains from its definite excretion. 



It is quite otherwise with the excretion of pure, or almost pure water. In 

 this case the actual removal of the water as such from the plant cannot be the 

 object to be attained, any more than the removal of water vapour can be the 

 most important aim in the process of transpiration. If we have rightly appre- 

 hended the significance to be attached to the rapid movement of nutritive salts, 

 it will be at once apparent that this exudation of water in the form of drops 

 must be regarded as a process which takes the place of transpiration in 

 situations where transpiration is, for other reasons, out of the question. Con- 

 tinuous transpiration in aquatic plants is impossible, and in their case excre- 

 tion of liquid water has been often observed (WEINROWSKY, 1899). [Compare 

 also POND, 1905 (Biological relations of aquatic plants to the substratum. 

 U.S.A. Fish Comm. Reports, 1903. Washington, 1905).] Moreover, transpiration 

 will be retarded temporarily in many land plants both during the night and 

 in the early morning, owing to the saturation of the air. It is just at these 

 times that the exudation of drops is conspicuous in such plants. 



