480 MECHANISMS FOR CONVEYANCE TO AND FRO. 



(sugar sheaths), and oohers again are the route for the transmitted starch (starch 

 sheaths). The sieve -tubes and bast parenchyma, on the other hand, convey 

 principally albuminous substances which are employed as constructive materials 

 for the growing and enlarging portions of the plant. 



This important relation of the soft bast to the growing organs explains 

 many remarkable phenomena, two of which must be briefly described here. 

 First, the surprising increase of growth in certain places which gardeners produce 

 by the operation of ringing. If two parallel circular cuts are made round a 

 growing branch of a tree through the whole thickness of cortex down to the wood, 

 and if the circular piece of cortex, together with the soft bast lying between 

 the two cuts is removed, the sap current in the soft bast from the upper portions 

 to the base of the branch is interrupted. The cut surfaces dry up; the route 

 down the soft bast is therefore closed, and the part of the branch lying below 

 the excision can no longer receive from the soft bast the materials necessary 

 to its further growth. The same result is obtained by passing a cord tightly 

 round the young leafy branch of a tree at some spot, say about half-way up. 

 In this way all the soft tissues which lie outside the wood, including the soft 

 bast, are compressed, the sieve-tubes and tracts of cells of the bast parenchyma 

 tightly squeezed together, and the conduction of sap by them to the base is 

 rendered impossible by the strangling cord. The ascending current of water 

 and dissolved food-salts, in the deeper-lying firm wood, flows on unimpeded in 

 either case. The green foliage -leaves can thus decompose carbonic acid and 

 manufacture organic substances above the circular cut or ligature; these pro- 

 ducts are then conducted away; the albuminous substances enter the soft 

 bast, but only travel as far as the place where the cut has been made or the 

 ligature been tied. They can no longer pass these places, and consequently 

 the plastic albuminous materials become dammed up above the " ring " or ligature, 

 and a particularly luxuriant growth of all these parts results. Fruits which 

 develop from the blocked-up region of the branch sometimes enlarge to an 

 extraordinary degree, and become almost twice as heavy as they would have done 

 had the operation not been performed. 



The following phenomenon is also explained by the fact that the passage of 

 plastic albumins takes place in the soft bast. If a branch of a willow, e.g. of 

 Salix purpurea, be cut off and the entire cortex down to the wood be removed 

 from the lower third of the branch, and the branch so treated be then plunged 

 half-way into a vessel of water, after a time it will send out roots into the 

 water. But these differ from one another very much according as to whether 

 they arise from the lower stripped portion of the branch or from higher up 

 where the cortex has not been removed. The roots developed from the stripped 

 portion are few and remain very short; those springing from the upper thickening 

 portion of the willow branch, where the cortex is intact, are, on the contrary, 

 abundant and elongated, since they can utilize the plastic juices above the place 

 where the cortex, together with the soft bast, has been removed. 



