200 



BOTANY 



PART 1 



its surroundings, transpiration is still possible even in an atmosphere saturated 

 with aqueous vapour. In the process of exudation the plant has further a means 

 of giving off water even after transpiration has completely stopped. 



Exudation {^^') 



The discharge of water in a liquid state by direct exudation is 

 not of so frequent occurrence as its loss by transpiration, but is found 

 under special conditions, viz. Avhen the plant is saturated Avith water 

 and the air is saturated with water vapour. Early in the morning, 

 after a warm, damp but rainless night, drops of water may be 

 observed on the tips and margins of the leaves of many of the plants 



of a meadow or garden. These drops 

 gradually 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 Avater exude 

 from the leaves themselves. They are 

 discharged near the apex of the leaves 

 of the Indian Corn, but in the case of 

 Alchcmilla from every leaf -tooth, and of 

 the Nasturtium from the ends of the seven 

 main nerves (Fig. 187). The drops dis- 

 appear as the sun becomes higher and 

 the air warmer and relatively drier, but 

 can be induced artificially if a glass bell- 

 jar be placed over the plant, or the 

 eva2)oration in any way diminished. The 

 excretion of drops from the leaves can 

 be brought about by artificially forcing 

 water into cut shoots. The drops are pressed out from special water- 

 stomata (p. 115), or in other cases through the ordinary stomata, 

 or from clefts in the epidermis (apical clefts). Sometimes they are 

 secreted by specially constructed hairs (p. 119), and in Datura the 

 water is excreted through the walls of the ordinary epidermal cells. 

 All such water-excreting organs are termed hydathodes. 



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

 of water may often be seen to fall, within short intervals, from tlie tips of the large 

 leaves. From the leaves of Colocasia ny mphac folia the exuded drops of water are 

 even discharged a short distance, and 190 drops may fall in a minute from a single 

 leaf, while -^^ litre may be seci'cted iu the course of a night. Again, in unicellular 

 plants, especially some Moulds, the copious exudation of water is very evident. 

 The water in this case is pressed directly through the cell-walls, and in some cases 

 also, as is the case in water plants, through the easily permeable cuticle. 



Fia. 1S7. — Exudation of drops of water 

 from a leaf of Tropaeolum mujns. 



Since the excretion of water in the liquid form can occur when 



