92 LECTURE VI. 



plant, set up by the active absorption going on in the organs 

 (rhizoids) which here perform the functions of roots. Again, 

 if care be taken to prevent evaporation, it will be found that 

 drops are formed on the margins and at the apices of the 

 leaves, especially the younger ones, of many plants, such as 

 Grasses, Aroids, Alchemillas, Saxifrages, etc. That the forma- 

 tion of these drops depends upon the forcing of water into 

 the cavities of the vessels by the root-pressure is shewn by 

 the fact that if the stem be cut off and placed in water, no 

 more drops will appear on the leaves. If, in this experiment, 

 the cut stems be allowed to remain for some days in water, 

 they will in many cases produce adventitious roots: when this 

 takes place the formation of drops on the leaves at once 

 recommences. Again, Moll has shewn that if the root- 

 pressure be replaced, in the case of a cut-off branch, by the 

 pressure of a column of mercury, an exudation of drops will 

 take place. 



Some idea of the quantity of water which thus exudes in the form of 

 drops is afforded by the following observations. Williamson obtained 

 half-a-pint of fluid in one night from a leaf of Caladium distillatorium. 

 Unger obtained from Richardia jEthiopica, in one case, 26 '5 grms. from 

 six leaves in eleven days, and jn another, 36 grms. from four leaves in 

 ten days : from a leaf of Colocasia antiquorum (var. Fontanesii) Duchartre 

 obtained in one night (August) 12 grms., in the following night 13*1, and 

 later (commencement of September) 14-35 grms. ; in other instances he 

 obtained as much as 22'6grms. On several occasions Duchartre observed 

 the formation of 120 drops in one minute. 



Those leaves which exhibit this exudation of drops frequently have 

 specially modified apertures, termed water-pores (see Fig. 19, A\ through 

 which the drops come to the surface, and these pores are placed singly 

 or several together over the termination of a fibrovascular bundle, which 

 may or may not be glandular. In other cases the drops pass through 

 the ordinary stomata, and in others again the water exudes, not through 

 any special openings, but simply through the walls of the epidermal cells. 



It can be readily observed that the liquid which is poured 

 out by a bleeding stem escapes from the orifices of the vessels 

 of the wood, and it is also in these channels that it is con- 

 veyed to the leaves on which drops are formed. We must now 

 endeavour to become acquainted with the mode in which the 

 fluid absorbed by the root-hairs obtains access to the vessels, 



