306 ORGANS FOR SECRETION. [BOOK 11. 



The same author observed a similar phenomenon in the 



leaves of Canna angustifolia, indica, and latifolia. The 



secretion of watery fluid takes place in these plants, not from 



the point of the leaves as in Calla, but from the points of the 



parallel ribs, which terminate at the margin of the leaf; and 



generally in greater abundance from those nearest the apex 



of the leaf. From these points of the principal veins, where 



they lose themselves in a delicate network, towards evening 



and at night there exudes imperceptibly a clear watery fluid, 



which collects in drops and patches on the upper and on the 



under surface of the lamina, occasionally running from 



it as copiously as from the point of the Calla sethiopica. 



Temperature seems to have some connection with this 



excretion, which is rather promoted by the growth of the 



leaves, but ceases when the plant puts forth flowers. 



Observations of a similar kind have been published by 



Rainer Graf, (Flora, 1840, p. 432.) In the Impatiens 



nolitangere he observed drops of fluid on the cotyledons, and 



in the first leaves at the edge whilst still folded together in 



the bud. As soon as the leaves attain their full size, the 



drops appear on the crenatures, at the point of each sepal, 



until the capsule begins to swell, at the point of the bracts, 



and also on the flowers. They appear there at the middle 



tooth of the upper arched petal, and at the point of the lower 



valve-like calcarate petal. The drops are largest on the 



cotyledons; those on the leaves follow next ; and there they 



are always larger at the points of the ribs than at the points 



of the secondary nerves. The drops, which in other respects 



consist of perfectly tasteless and scentless water, usually 



appear within ten or twenty minutes after rain or watering 



the plant. It appears that the vessels which are situated in 



the ribs of the leaves carry the sap rapidly from one place to 



another, conducting it finally to those points where it is 



necessary for the nourishment of the plant, and throwing it 



oft them when in excess, or when carried thither faster than 



it can be absorbed. 



In these cases it can scarcely be said that special organs are 

 formed for carrying on the office of secretion. In many 

 plants, however, glands are evidently provided for such a 



