January, 1912] 



5 



Saps and Exudations. 



thesis; I may point out that the eon- 

 version of caoutchouc into assimilable 

 carbohydrates in situ is extremely un- 

 likely. This is apparent from an ele- 

 mentary consideration of such a con- 

 version from the chemical standpoint. 

 Caoutchouc consists of the elements 

 carbon and hydrogen only, while the 

 assimilable carbohydrates contain in 

 addition the element oxygen, and in 

 such proportion that for every two 

 hydrogen atoms there must be one oxy- 

 gen atom. A simple calculation shows 

 that caoutchouc would require about its 

 own weight of oxygen, and the question 

 arises— whence could this large amount 

 of oxygen be derived. If derived from 

 other constituents these would be 

 correspondingly reduced. The only 

 possible source of such oxygen would be 

 the air, and it follows that such changes 

 would be confined to the latex in the 

 leaves and could not affect the latex in 

 the vessels of the cortex. On the other 

 hand, resins are fairly rich in oxygen. 

 Or if the caoutchouc be regarded as a 

 waste produce, its formation entails the 

 liberation of large quantities of oxygen. 



In the present note it is not intended 

 to put forward any new theory cf the 

 function of the latex in the plant, but 

 to formulate an aspect of the question 

 which renders intelligible the primary 

 function of the caoutchouc and resins in 

 the latex. 



These inert constituents are always 

 present in a very finely divided form as 

 minute globules, and the chemico- 

 physical functions of matter in this 

 state appear to have been lost sight of.* 

 I am speaking of the functions or 

 properties which invariably accompany 

 matter in a finely divided form. All 

 work up to now has centered on the 

 caoutchouc (and resins), and experi- 

 ments on yields have been restrict- 

 ed to the percentages of caoutchouc 

 without any regard to the proportion 

 of the other constituents. 1 refer to 

 the nitrogenous matters, carbohydrates, 

 salts, &c. We have no information as 

 to the variation in yields of these latter 

 substances, and, except for a few 

 analyses, we have no information at 

 all on this subject. Even where analyses 

 are given, we seldom find exact data 

 as to age of the trees, previous tapping 

 history, &c., so that the analyses are of 

 little value for our purpose. 



To return to the globules of caou- 

 tchouc (and resins), let us enquire what 



* Since writing the above my attention has 

 been called to Dr. Hans Molisch's 'Stutlien 

 uber MilchsafV page 81, where the possible 

 surface action of the globules is mentioned. 



are the characteristics of these sub- 

 stances in the finely divided form in 

 which they occur, — what properties, 

 chemical and physical, must necessarily 

 accompany minute elobules in suspension 

 in an aqueous medium lightly charged 

 with water-soluble substances. 



It has long been known that the con- 

 centration of molecules on surfaces is 

 denser than in other parts of a medium, 

 and to this change in concentration the 

 term adsorption is given ; the adsorption 

 of ammonia gas by charcoal is a common 

 text-book example. The ammonia be- 

 comes condensed in a concentrated form 

 on the surfaces of the particles of 

 charcoal. Similarly salts may be adsorb- 

 ed from solution. This is the reason 

 why it is so difficult to remove the last 

 traces of soluble salts when washing a 

 precipitate, and why a proportion of 

 the salts is always retained by the soil 

 in spite of the heaviest rains. This 

 phenomenon is dependent on the area of 

 active surface in which change in con- 

 centration takes place,— it is therefore 

 the more marked, the more finely divided 

 the substance. The action is therefore 

 particularly noticeable with the fine 

 particles such as soils are composed of, 

 and it must also play a prominent part 

 in latex, where the globules are very 

 small and the surface exposed to the 

 aqueous medium or continuous phase is 

 very large. Assume then that we have 

 an aqueous medium with small quant- 

 ities of dissolved carbohydrates, salts, 

 &c, and that it is possible to add to this 

 solution a quantity of caoutchouc or 

 resin in small globules of, say, 0*5 microns 

 to 3 microns in diameter such as are 

 commonly found in latices. The im- 

 mediate effect would be the withdrawal 

 from the solution of part of the 

 dissolved substances, which would be 

 adsorbed in the surfaces of the 

 caoutchouc globules. With the same 

 globules the amount adsorbed will 

 depend on the concentration of the 

 soluble substances in the aqueous 

 mediftm. If the concentration be in- 

 creased by dissolving moieof the soluble 

 materials, part only of these would re- 

 main in solution and the remainder 

 would be adsorbed ; or if part of the 

 soluble materials were removed so as to 

 reduce the concentration of the solution, 

 this would be to some extent counter- 

 balanced by part of the adsorbed sub- 

 stance being given up to the solution. 

 The effect of the presence of the inert 

 caoutchouc or lesin globules is to in- 

 crease the capacity of the latex for 

 retaining its constituents. For the same 

 concentration of these constituents the 

 capacity of the latex for holding certain 



