100 COMPENDIUM OF GENERAL BOTANY. 



conducting or as 2X^\\.m!&Ci.-storing (Frank). From tlie anatomical 

 structure of sieve-tubes we may, however, safely assume that they 

 are qualified to permit the mass movement of undissolved albumin- 

 oid substances (see above). The old and well-known girdling ex- 

 periments teach that the substances necessary in the formation of 

 organs, that is, albumen and carbohydrates, are checked in their 

 course when the entire bark is removed. The question whether 

 the two plastic substances each require a special path, that is, pri- 

 mary cortex for the one and secondary cortex (leptome) for the 

 other, or whether the conditions are otherwise, is still unsettled. 

 Frank is inclined to assume that the primary and secondary 

 cortex, without the sieve-tubes, is chiefly employed in the conduc- 

 tion of carbohydrates and amides (the latter are considered as cir- 

 culatory forms of albumen). The opinion that the contents of the 

 sieve-tubes (especially albumen) are essentially serviceable in the 

 camhiuin for the formation of tissues is doubtless correct, but it 

 seems in exact opposition to the principles of physiological anatomy 

 to assume that the elongated elements of the sieve-tubes simply 

 serve the function of a storage-tissue in \\'hich albumen remains at 

 rest until required for use in the immediate vicinity. Frank's 

 process of reasoning will no doubt bring us nearer the truth. Ac- 

 cording to this authority, the soluble amides and the carbohydrates 

 circulate in the parenchyma. From the amides are formed mu- 

 cous (undissolved) albuminous substances ; in the winter these col- 

 lect in the sieve-tubes and remain at rest. At the time of cambial 

 activity this mucous mass in the sieve-tube system oscillates, due to 

 the bendings of the stem and changes of the turgor-force accompa- 

 nied by a gradual reabsorption of the albumen. Frank's studies 

 (and those of his pupils) may perhaps soon lead to the recognition 

 of the fact that the sieA'e-tubes really form a storage system, but that 

 they are also admirably adapted to permit mass-movement in a longi- 

 tudinal direction during active assimilation. 



Since we have referred to " girdling," we may also mention 

 the well-known experiment which is made as follows : aleaf-bear- 

 ing twig (of wdllow) is girdled a little above its leafless, lower end 

 and the cut end placed into moist earth up to a point somewhat 

 above the girdle. IS'o roots, or but very few, appear below the 

 ofirdle, while numerous roots and callus are formed above it. A 

 current of formative substances passes from the assimilating leaves 



