152 



HUTCHINSON'S POPULAR BOTAXY 



flowers, is presumably due to electrical conditions of the atmosphere, 



and, in that case, it ought perhaps to be classed among abnormal 



phenomena. 



But it is time to conclude this long digression, and to return to our more 



immediate subject the sap of plants. 



The true sap, which conveys the elaborated food material from the 



leaves to the root, etc., is very different from the crude, thin, watery sap 



which ascends from the root to the leaves. 

 A curious fact, illustrative of this differ- 

 ence, is, that the latter is nearly or quite 

 harmless in those plants whose proper 

 juices have the most virulent properties. 

 Thus, according to Carpenter, " the in- 

 habitants of the Canary Islands draw off 

 the ascending sap, which serves as a 

 refreshing drink, from the interior of 

 the stem of Euphorbia canarievisis, a tree 

 of which the descending sap is of a very 

 acrid nature, resembling that of the 

 Common Spurge (E. peplus} of this 

 country, but much 'more powerful." It is 

 important to bear this distinction clearly 

 in mind. The crude sap ascends, as we 

 had seen, chiefly by way of the wood 

 elements of the vascular system: while 

 the elaborated sap, avoiding the wood 

 elements, passes down the sieve-tubes, 

 the cellular tissues of the bark,* and, 

 possibly, the laticiferous vessels, though 

 it is now a question whether the latter 

 play an important part as distributors. 



Thus we have an ascending and a de- 

 scending, a crude and an elaborated sap, 

 and each pursuing independent routes 

 through quite distinct parts of the plant. 

 When the experiment has been tried of 

 removing a ring of bark from a tree 



* The elaborated sap containing the nitrogenous 

 organic substances (i.e. the soluble results of proteid 

 FIG. 189.-GREATER DODDER conversion) descends by way of the sieve-tubes, 



(Cuscuta europcea). an< ^' perhaps, the laticiferous vessels, while that 



containing the nan-nitrogenous organic substances 



A twining leafless parasite that commences growth . , , , . 



in the earth, but soon attaches itself to its victim (sugar, etc.) passes downwards through the par- 



by suckers, and then gives up its roots. enchyma. 



