SECT. II 



PHYSIOLOGY 



193 



able height. Vigorous coniferous shoots absorb water through the out end 

 with a force of suction equal to one atmosphere, and are thus able to raise the 

 mercury to a height equal to the barometric pressure (760 mm.). The complete 

 exclusion of the external atmosphere is absolutely requisite for the existence of 

 such a suction-force, a condition actually fulfilled in the water-courses of plants. 



II. Exudation of Water. — The discharge of water in a liquid 

 state by direct exudation is not of so frequent occurrence as its loss 

 by evaporation in the form of vapour. 

 Early in the morning, after a damp night, 

 drops of water may often be found on 

 the young leaves of Indian Corn, and 

 also on the leaves of Alchemilla and the 

 Garden Nasturtium. These drops gradu- 

 ally increase in size until they finally 

 fall off and are again replaced by smaller 

 drops. These are not dew-drops, although 

 they are often mistaken for them ; on 

 the contrary, these drops of water exude 

 from the leaves themselves. They are 

 discharged near the apex of the leaves 

 of the Indian Corn, but in the case of 

 Alchemilla from every leaf-tooth, and of 

 the Nasturtium from the ends of the 

 seven main nerves (Fig. 180). The PlG - _ lso - 

 drops disappear as the sun becomes 

 higher and the air warmer and relatively drier, but can be produced 

 artificially if a glass bell-jar be placed over the plant, or the 

 evaporation in any way diminished. Whenever plants become 

 overcharged with water through the activity of the roots, it is 

 discharged in drops. These are pressed out of special water- pores 

 (p. 95), and sometimes even from the stomata and clefts in the 

 epidermis ; while in Datura they have even been observed to exude 

 directly through the walls of the epidermis. It is possible to cause 

 similar exudations of water by forcibly injecting water into a cut 

 shoot. 



Such exudations of water are particularly apparent on many Aroids, and drops 

 of water may often be seen to fall within short intervals, sometimes every second, 

 from the tips of the large leaves. From the leaves of a species of Colocasia the 

 exuded drops of water are even discharged a short distance. In Spathodea, a 

 tropical member of the Bignoniaccae, the space enclosed by the calyx, in which the 

 young floral organs are developed, is filled with water. Again, in unicellular 

 plants, especially some of the Fungi (Mucor, Pilobolus, Phycomyces), the copious 

 exudation of water is very evident. The water in this case is pressed directly 

 through the cell walls. 



The organs for the discharge of water, which Haberlandt has collec- 

 tively termed hydathodes (pp. 91, 99, 114), in some instances, like 







Exudation of drops of water 

 from a leaf of Tropaeolum majus. 



