3»2 



certain number of facts and observations which may direct experi- 

 menters in their researches. 



In the case for example of Landolphia Heudelotii Dec. which 

 supplies the greater part of the caoutchouc exported from Senegal, 

 the Soudan and of Guinea it is easy to see in a transverse section 

 of the vine that the laticiferous vessels are chiefly found in the 

 inner half or the baTk but that they are nearly completely absent 

 from the centre part of the bark as well as in the zone nearest the 

 wood. To get at these vessels therefore it is not necessary to pe- 

 netrate as far as the wood. The vessels in Landolphia Heudelotii 

 Dec. are elongate tubes branched and anastomosing with a dia- 

 meter varying from 30 to 40 thousandths of a millimeter. They 

 extend chiefly in the direction of the length of the stem but as I 

 have just said, branch and these branches run more or less obli- 

 quely. 



A transverse section therefore of the bark of a given length and 

 depth will cut across a certain number of the vessels from which 

 the latex will escape while a longitudinal- section of equal length 

 and depth will only cut through a much smaller number (a figure 

 to shew this is given in the text which we are unable to reproduce 

 but the description is intelligible without thi?. Ed.) It is however 

 easy to show this at least in Landolphia Heudelotii in the fol- 

 lowing manner. One knows, (and this is what the extraction of 

 rubber from the dry bark is based on) that in this vine the latex 

 dries of itself in the laticiferous vessels of the bark so that each 

 vessel contains a very thin strand of rubber. If then one breaks 

 transversely a piece of dry bark and carefully separates the frag- 

 ments, one sees them joined by the thin filaments of rubber 

 representing the number of vessels in the section. If the bark 

 is broken longitudinally (/ e. parallel to the direction of the stem) the 

 number of threads of rubber drawn out is very much less, showing 

 that the number of vessels cut across by a transverse section is far 

 greater than those cut across by a longitudinal section. I have 

 besides verified the fact in a young Castilloa, and Willes (Morris 

 Cantor Lecture published by the Society of Arts April 1899,) has 

 shown that in Heveas cultivated in the gardens at Heneratgoda, 

 Ceylon> other things being equal incisions made obliquely at an 

 angle of 45 degrees produce twice as much latex as vertical ones. 



Transverse sections offer another advantage in the matter of 

 collecting the latex. On account of the constant growth of the 

 woody cylinder surrounded by bark, the bark not following this 

 growth is stretched more and more, like a too tight coat round a 

 stout body. It is this tension of the bark which produces the 

 longitudinal cracks so characteristic of the oak of our country for 

 example. If one takes off a transverse band of bark from the 

 trunk of a tree and tries afterwards to put it back in the same 

 place it was taken from it will be found that the ends will no long- 

 er meet. 



It is just this tension which causes the escape of the latex which 

 without this, capillarity would keep in the laticiferous vessels. 

 Now in making a transversa cut one does not reduce the tension 



