METABOLIC PROCESSES IN THE PLANT. 357 



and consequently colouring blue litmus red. If we again touch 

 our cut surface with red litmus paper, we shall soon find that the 

 whole surface of the paper touching it is no longer turned blue, but 

 only particular parts, those namely which come into contact with 

 the vascular bundles. If we now touch the cut surface with blue 

 litmus paper, this becomes red except in certain places. Directly 

 after cutting the stem of Cucurbita there exudes a mixture of sap 

 preponderatingly alkaline in reaction. Proceeding, however, as 

 above, we readily make out that the sap of the parenchyma in 

 Cucurbita, as in other plants, is acid in reaction, while that of 

 certain tissues of the fibro-vascular bundles, viz. those of the soft 

 bast, have an alkaline reaction. In other plants the conditions 

 are similar, but it is not so easy to determine them with 

 certainty. 1 



We now prepare a transverse section from the hypocotyl of 

 Cucurbita Pepo, using alcohol material. In most plants soft bast 

 occurs only on the outside of the vascular bundles, but here it is 

 to be found both on the outside and the inside of the wood of the 

 bundles. If we test for proteids in the manner given in 94, it 

 is found that the elements of the soft bast contain large quantities 

 of- proteid substances. The alkaline mucilage, rich in proteid, 

 which escapes on cutting the Cucurbita stems, is present in par- 

 ticularly large quantities in the sieve tubes of the soft bast, 

 characteristic elongated elements which are divided by trans- 

 verse walls (the sieve plates), pierced by numerous pores. In the 

 sieve tubes parietal protoplasm is present, and they are filled with 

 an alkaline mucilage rich in proteid, which by means of the sieve 

 pores can pass from one member of the sieve tube to another. 

 And in fact such a movement of the mucilage must tako place in 

 the uninjured plant, from the same causes as bring about the 

 escape of the mucilage when the plants are injured. The sieve 

 tubes, viz., are subjected to the pressure of the turgescent 

 parenchyma in their neighbourhood. Hence their contents can 

 be passed on to places of less pressure, especially to the very 

 young parts of the plant, and we see therefore that the sieve 

 tubes function as organs for the translocation of proteids, which 

 is in complete harmony with the results obtained from the ringing 

 experiments. 



The circulating proteid in the mucilage of the sieve tubes 

 moves en masse, and can be transported from one place in the 

 plant to others often far removed. We must however make our- 



