148 PHYSIOLOGY OF NUTRITION 



of this. A portion of a plant was allowed to grow in a water-saturated atmos- 

 phere, under a bell-jar, while the remainder was exposed to natural condi- 

 tions. The ash content in the leaves grown in the moist atmosphere was 

 lower than that of the other leaves, the former being only 13 per cent., while the 

 latter was 21.8 per cent., of the total dry weight. 1 " 



§5. Movement of Organic Substances. — Malpighi's girdling experiment, 

 already described (page 133), indicates that organic substances move through 

 plant stems only in the cortex. This region, however, includes many different 

 kinds of tissue and the question arises whether the movement here considered 

 occurs equally throughout the cortex or only through special parts of it. Han- 

 stein 1 carried out a series of experiments in this connection and found that the 

 removal of a ring of cortex did not always stop growth in the region below the 

 lesion. Anatomical study of the plants that were not injured showed that some 

 of these possessed vascular bundles in the pith as well as in the ring of vessels 

 always found in dicotyledonous plants, while others possessed no collateral 

 bundles and had only bicollateral ones. Girdling had no effect upon the growth 

 of monocotyledonous plants. Hanstein concluded, therefore, that this dif- 

 ference between different plants, in regard to the effect of girdling, is due to the 

 fact that all the sieve-tubes are removed in the girdling of most dicotyledonous 

 plants, while only a part of them are removed in those dicotyledons that have 

 vascular bundles in the pith, and in monocotyledons with bicollateral bundles. 

 Sieve-tubes are therefore the main channels through which the movement of or- 

 ganic material occurs. By virtue of their anatomical structure these tubes are 

 better suited for this movement than are any of the other tissues of the cortex. 

 This conclusion does not at all exclude the possibility that organic substances 

 may move by diffusion through any other living cells, especially through the very 

 small pores by which many cell walls are perforated. A peculiarity of the move- 

 ment of organic materials is that it is regulated exclusively by the activity of 

 living cells and that it is a result of this activity. In other words, this move- 

 ment is controlled by internal conditions. External conditions affect transloca- 

 tion only as they affect the life-processes of the cells in general. With the upward 

 movement of the soil solution it is quite different, for, as has been seen, this is 

 very largely dependent upon such external conditions as light, humidity, wind, 

 etc. The movement of the soil solution has been somewhat thoroughly investi- 

 gated in its general aspects, but our knowledge of the translocation of organic 

 materials rests upon only a few well-known facts and is largely hypothetical. 



The movement of organic materials has been extensively studied in connec- 



1 Hanstein, Johannes, Versuche iiber die Leitung des Saftes durch die Rinde und Folgerungen daraus. 

 Jahrb. wiss. Bot. 2: 392-467. i860. 



" But see: Hasselbring, Heinrich, The relation between the transpiration stream and the 

 absorption of salts. Bot. gaz. 57: 72-73. 1914. Hasselbring's conclusion is the direct 

 opposite of the one reached by Schlosing. The question as to what rates of transpiration 

 are necessary to elevate the requisite amount of salts in tall plants deserves further atten- 

 tion at the hands of experimenters. It appears clear enough, on a priori grounds, that some 

 transpiration must generally give better growth than none at all, but the rates generally 

 experienced by ordinary plants are probably much higher than the optimum. — Ed. 



