August, 1909.] 



139 



Plant Sanitation. 



limited, i.e., that the effect does not 

 spread continuously, argues against any 

 fungus agency. 



The facts detailed above support the 

 statements that (a) the rubber pads can 

 form independently of the use of the 

 pricker, (6) the bark dies in patches in 

 consequence of* scraping, (c) the rubber 

 pad is formed after, not before, the 

 death ot the bark. 



Exception has been taken to the state- 

 ment recently made in this column that 

 the rubber in Hevea is a " waste pro- 

 duct." There have been practically 

 no experiments or observations in this 

 direction, the majority of investigators 

 having occupied themselves with the 

 question of obtaining the maximum 

 amount of rubber from the tree, without 

 troubling to consider how and where 

 it was formed. The following observa- 

 tion tends to support the " waste pro- 

 duct" theory. If a naturally shed 

 Hevea leaf is taken, and a thin layer of 

 the midrib on the back of the leaf is 

 peeled off slowly, strands of rubber will 

 be seen stretching across the angle 

 between the strip which is being peeled 

 off and the remainder of the midrib. 

 This shows that when the leaves are 

 shed naturally, e.g., at the initiation of 

 the " wintering" stage, the rubber which 

 they contained remains in them. The 

 same result may be obtained with rather 

 more trouble by extracting the rubber 

 from the fallen leaves by means of 

 carbon bisulphide. Now, according to 

 Sachs 'investigations, before a tree sheds 

 its leaves, all the potash, phosphoric 

 acid, starch, etc-, in them — in fact every- 

 thing which can be of further use to the 

 plant, — is transferred from the leaves to 

 the stem. The dead leaf retains only 

 waste products. The fact that the 

 rubber is cast off in the dead leaf tends 

 therefore to support the view that it is 

 also a waste product. 



It is generally believed that all the rub- 

 ber which is formed in the stem of 

 a tree accumulates in the laticiferous 

 tissue until the planter chooses to tap it ; 

 that if he does not tap until the tree is 

 eight years old, he will obtain all the 

 rubber which was in the tree when it 

 was, say, six years old, plus the amount 

 which has formed in the additional two 

 years. From a botanical standpoint 

 this is improbable. After the stem has 

 passed its green stage, it acquires the 

 normal secondary cortex with a dead 

 corky layer on the outside ; and as it 

 grows older, this corky layer increases 

 in thickness. But the corky layer is 

 foimed from the laticiferous layers, 

 This is readily seen when a tree is pared ; 

 the exposed laticiferous tissue is then 



rapidly covered by a new corky layer 

 which is obviously formed from it. 

 Further, if a tree is tapped by the full 

 spiral method with spirals one foot 

 apart, and the tapping is stopped after 

 a breadth of six inches has been cut 

 away along each spiral, the original 

 bark left between the spirals will in 

 some cases scale off in flakes down to 

 the level of the renewed bark. I have 

 taken off scales of brown bark, ten 

 inches long, which were formed between 

 two spirals. Now, the brown corky 

 layers, and the scales just referred to, 

 were originally laticiferous ; and the cur- 

 rent belief assumes that this latex was 

 transferred inwards to the inner bark 

 when the corky layer was formed. But 

 if the brown scales of corky bark are 

 pounded up in a mortar, and then ex- 

 tracted with carbon bisulphide, it is 

 found that they contain an appreciable 

 quantity of rubber. Just as in the case 

 of the leaves, therefore, rubber is dis- 

 carded with the brown bark, and thus 

 the current belief is shown to be incor- 

 rect. When laticiferous bark is convert- 

 ed into corky bark, the latex which it 

 contains dries up, and the rubber is left 

 in the dead layer. Some of the rubber 

 which was in the tree at the age of six 

 is undoubtedly rendered unavailable 

 before the tree is eight years old. The 

 amount might be estimated if the rate 

 of growth of bark were known ; it 

 cannot be a very considerable quantity 

 when the tree is young. 



It appears therefore that the tree is 

 always discarding rubber as well as 

 manufacturing it, the balance being of 

 course in favour of the latter process, 

 This obviously contradicts the idea that 

 the rubber from a six-year old tree is 

 itself six years old, or that rabbet 

 extracted from an eight year old tree is 

 necessarily older than that extracted 

 from a six year old tree. However, this 

 contradiction is superfluous, for it is 

 evident that in any tree most of the 

 latex is derived from near the cam- 

 bium in the present systems of tap- 

 ping, and that that latex is the most 

 recently formed. 



It must be pointed out that the experi- 

 ment referred to here is qualitative only, 

 i.e., it only proves that rubber is dis- 

 carded with the dry brown bark. Obvi- 

 ously it is necessary to compare the 

 actual weights of rubber in equivalent 

 volumes of laticiferous and brown bark 

 respectively, before it can be asserted 

 that all the rubber in a given thickness 

 of laticiferous bark is rendered unavail- 

 able when that bark is transformed into 

 the corky brown layer. 



This is not intended to serve as an 

 argument in favour of early tapping. 



