79 



bored into a tree in winter and the part be rapidly warmed, 

 or if the temperature of the air be rising, the increase of 

 temperature suddenly expands the air-bubbles mixed with 

 the fluid, and causes the fluid to be driven out of the stem 

 where it can find an opening. When the part is cooled again, 

 the air contracts, and fluid is absorbed. This is sometimes con- 

 founded with the bleeding of plants, which we have seen is due to 

 root pressure only. The exudation or movement of the liquid 

 by increase of temperature can only occur in the winter and early 

 spring, before the leaves unfold and transpiration begins, be- 

 cause it is only at that period that air and liquid exist together 

 in the cavities of the vessels. 



Rather a striking evidence of the existence of this root 

 pressure, and a phenomenon well worthy of note, is the secre- 

 tion or excretion, under certain circumstances, of actual drops 

 of water, as water, by the leaves of certain plants. In the 

 process of transpiration the water always passes off from the 

 plant in the form of vapour, but in certain other cases we find 

 that drops of water are exuded. This pouring out of water is due 

 to the osmotic activity of certain structures which have received 

 the name of water glands. As regards the position of these 

 glands on the leaf, it seems to be the rule that they occur on 

 the margin of the leaf. Exceptions to this statement are 

 afforded by several species of Crassulas ( C. cordifolia, C. arbor- 

 escens, and C. portulaced), where they occur distributed over 

 the surface of the leaf. In leaves with an entire margin they 

 usually occur only at the apex, i. e., the extremity of the main 

 fibro- vascular bundles, as in the species of Azalea, Myosotis, 

 Arum, Caladium distillatorium, Musa, Richardia, Collocasia, 

 Zea Mais, and Hordeum vulgar e. In leaves whose margins are 

 cut or indented there may be a gland at the apex of each tooth, 

 as in the Chinese Primrose {Primula sinensis), in Fuchsia glo- 

 bosa, and in the common Lady's Mantle {Alchemilla vulgaris) ; 

 also, in Bryophyllnm calycinum; in the indentation between 

 two teeth, as in Crassula spathulata; or even on the sides of 

 the tooth, as in Senecio petasitis. In Tropceolum majus they 

 occur only on the upper surface of the leaf jiear the margin. 



