98 METABOLISM 



their use of water, and by reducing transpiration by special methods which are 

 naturally adapted to the special conditions under which they live. It is of 

 interest to note that some of them have discovered a means, i. e. by hyda- 

 thodes, of getting rid of excess of salt, and thus of preventing a superabundant 

 accumulation of sodium chloride in their tissues (compare p. 58). 



Our acquaintance with the causes which determine why many plants prefer 

 soils rich or poor in lime is at present much less complete. It is obvious 

 that the occurrence of plants in either situation is not directly connected with 

 the need of the plant for calcium as a food-stuff, for no soil is so poor in calcium 

 that it cannot obtain all that it requires from it. The so-called ' calciphobous' 

 plants require calcium just as much as the * calciphilous ' plants, and they do, 

 as a matter of fact, absorb considerable quantities of it from the slate or primi- 

 tive rock on which they live. The solution of the question is rendered all the 

 more difficult inasmuch as one and the same species cannot live equally well 

 in all places. Only a few plants appear constantly to avoid lime soils, e. g. 

 sphagnum and certain other aquatic mosses, the majority of Desmidiaceae, and, 

 among Phanerogams, Sarothamnus scoparius, Castanea vesca, and Pinus pinaster. 

 In regard to the last-mentioned plant, Vallot (1883) has made many investi- 

 gations which show how exclusive it is in its choice of a soil. Generally speak 

 ing, soils in which it is said to thrive, and which contain more than about 

 3 per cent, of lime, are found on closer investigation to exhibit local conditions 

 (e. g. oases of rock poor in lime) which render its existence possible. The experi- 

 ments of Bonnet (Vallot, 1883, p. 202), made at Dijon, are of great interest, 

 confirmed, as they have been, by Mangin (according to Roux, 1900, p. 131) 

 at Besan^on. The chestnut flatly refused to grow there, and yet it was possible 

 to cultivate it easily when it had been grafted on the calciphilous oak. The 

 reason is probably that the root is injured by the excessive amount of lime in 

 the soil, and in support of the correctness of this view we may draw attention 

 to the frequently cited behaviour of Sphagnum, as well as of plants which occur 

 in its company, e. g. Drosera. After being watered with a solution containing lime 

 these plants come to grief very quickly ; solutions of lime salts act as poisons 

 to them. According to Ohlmann (1898), watering Sphagnum with calcium 

 sulphate and calcium nitrate is less harmful than treating it with calcium carbo- 

 nate. Ohlmann also says that a 0-05 per cent, solution of calcium carbonate 

 kills it in from twenty-two to thirty-two days, and that a 0'i5 per cent, solu- 

 tion is fatal in fourteen to twenty-four days. More recently, Grabner (1901, 

 p. 112) states that Weber cultivated Sphagnum on chalk successfully. A final 

 decision on this question must be postponed until further investigations have 

 been made. [Solms Laubach (1905, Die leitenden Gesichtspunkte einer 

 allg. Pflanzengeographie, p. 122) has offered a more accurate analysis of Weber's 

 results, whence it would appear that Sphagnum is certainly injured by lime, since 

 it can scarcely live in company with other plants which require that mineral. 

 Grabner's results obtained by cultivating Sphagnum on chalk apply only to pure 

 cultures.] Correns's (1896) observations on Drosera (compare Lecture 

 XXXVIII) also point to a directly injurious effect of lime. How limited our 

 knowledge is as to the function of lime in plant life, is evident from the investi- 

 gations which A. Engler has recently (1901) made on the distribution of 

 Castanea in Switzerland. Although we may consider this tree in general as 

 markedly ' calciphobous', it occurs, according to Engler, on sandstones and 

 marls rich in lime, and which possess a large proportion of potassium. 

 Engler believes that a great need for potassium, and not the absence of 

 calcium, determines in general the occurrence of Castanea on siliceous soils . It must 

 certainly not be overlooked that in the case of plants of lower grade such as 

 Sphagnum,\t maybe possible that calcium, as in the case of Algae, is not an essential 

 food-material, while in the case of the higher plants such a supposition is less 

 probable. 



