236 



LECTURE XIV. 



aid of the paper strips previously mentioned, the advantageous property of ascending 

 ■without becoming decomposed ; and the lithium is taken up by the cell walls not 

 more energetically than the water. It thus becomes a matter of ascertaining how high 



the lithium has ascended in one hour in 

 the stem, and thence into the leaves, when 

 a weak lithium solution of say 1-20/0 has 

 previously been offered to the roots for 

 absorption. The lithium has, moreover, 

 the advantageous property of being re- 

 cognisable in the interior of the plant, 

 even in the minutest quantities, in an 

 exceedingly simple manner, by means 

 of the well-known intense red line per- 

 ceived in the spectrum of incandescent 

 lithium vapour by means of a spec- 

 troscope. It thus suffices to burn in 

 a Bunsen's flame small chips of wood 

 from the stem of the plant investigated, 

 after this has been cut into small por- 

 tions, to recognise by means of the 

 spectroscope the presence of lithium ; 

 or the same observation may be made 

 with portions of the leaves of the plant 

 investigated. We owe the excellent 

 idea of allowing lithium salts, on ac- 

 count of their easy detection, to be 

 absorbed by plants, to Professor Mac- 

 Nab ; his researches however were so 

 uncritically conducted that it required 

 a protracted investigation on my part 

 to put to actual use for our pur- 

 pose the excellent properties of this 

 substance \ 



With all that has been said hitherto, 

 however, we have still obtained no idea 

 as to the form in which, and as to the forces by means of which, the water ascends in 



Fig. igS.—Piiius sylvestris: radial longitudinal section through 

 the wood of a vigorously growing branch, c* cambium wood-cells ; 

 a—e older wood-cells ; /, t', l" bordered pits of wood-cells, in order 

 of age ; st large pits where the cells of the medullary rays lie in 

 contact with the wood-cells (X 55°)- 



1 In my treatise, ' Ei7i Beitrag zur Kenntniss des aufsteigenden Wasserstromes in transpiriren- 

 den Pflanzen'' (Arb.des bot. Inst, in Wzbg., 1878, II. p. 148), I subjected to criticism the investigations 

 made by MacNab and Pfitzer with solutions of lithium, and showed that only plants with uninjured 

 roots can give satisfactory results. To quote some numbers here, for example ; in a branch of Salix 

 fragilis, with abundance of roots which absorbed the lithium solution direct, and not from the earth, 

 the lithium ascended 85 cm. high in i hour: in two plants of Zca Mays under similar circumstances 

 it ascended 30 and 42 cm. respectively. With plants rooted in the soil and watered with the 

 lithium solution, and placed favourably as regards transpiration, the lithium ascended, in i hour, 

 118 cm. in Nicotiana tabacum, 206 cm in Albizzia lophantha, 107 cm. in Musa sapientuni. 70 cm. in 

 Helianthus annuus, and 98 cm. in Vitis vinifera. That experiments with coloured solutions give 

 no certain results, I have insisted upon in my ' Experimental-physiologie,'' 1865, p. 217. 



