RESPIRATION AND OXYGEN. 11 



fleshy roots of carrot and the fibrous roots of Senecio during 24 hours. 

 The respiration of the latter was more than 6 times greater than that 

 of the former. The amount of C0 2 expired by plants was found to 

 be the greater the richer they were in protein and the more extended 

 surface they presented relative to their mass. When green parts 

 of plants were submerged they respired as aerial plants, but with the 

 difference that the amount of CO 2 was reduced because the medium 

 was poorer in oxygen. 



While Liebig (1858) was apparently the first to note the etching of 

 limestone and ascribe it to the action of roots, Sachs (1860, 1865) first 

 demonstrated experimentally that this was the case. He grew the 

 roots of corn on polished marble plates and found that they etched 

 the surface. He concluded that this was due to the excretion of 

 C0 2 from the roots, since roots grown in distilled water quickly 

 charged this with C0 2 . He also thought it possible that the corrosion 

 might be due to the acid cell-sap which saturated the cell-wall. His 

 early work was confirmed by experiments in 1860 and 1864, showing 

 the corrosion of the surface of dolomite, magnesite, etc. He as- 

 sumed that this might be produced by C0 2 , but also thought that it 

 might perhaps be due to the acid sap of the roots themselves. He 

 detailed at some length the studies of 1864 with several species of 

 plants on various surfaces. According to Mayen (1838, 1 :11), Molden- 

 hawer thought that the root-hairs secreted a sap which served to 

 dissolve any other material. 



Knop (1861, 1864) confirmed the results of Sachs in regard to root 

 excretion. He found that a considerable amount of C0 2 accumu- 

 lated during the growth of grasses in neutral solutions. In further 

 researches, proof was obtained that CO 2 was excreted by the roots 

 of growing plants and this was regarded as a device for increasing 

 the absorption of solutes. He concluded that all organs absorb 

 oxygen in producing C0 2 , and hence that the tissues of land plants 

 were filled throughout with air containing C0 2 . It was regarded 

 as probable that the C0 2 excreted by the root serves universally 

 for the solution of mineral substances in the soil, and as possible 

 also that other more permanent acids aid in this work. 



Nobbe (1865) showed that potato tubers respire during storage in 

 the winter and that the amount of starch was correspondingly 

 reduced. Fleury (1865) confirmed the results of Huber and Senebier 

 and of Saussure in showing that dry seeds begin to evolve carbon 

 dioxid soon after the absorption of water. 



Corenwinder (1867 : 63) grew roots of Cuphea attached to the 

 aerial parts in air with a known quantity of C0 2 in order to deter- 

 mine whether roots absorb the latter. He found, on the contrary, 

 that a considerable amount of CO 2 was exhaled in the case of a 

 number of roots. Similar experiments were made with cabbage and 

 with Eupatorium with the same results. The author found, more- 



