HYDATHODES 147 



above the bundle-end breaks down completely, so that the 

 escaping water passes directly to the exterior. 



Hydathodes can often be recognised at a very early stage, 

 and are probably most active in the young leaf, which develops 

 in an almost saturated atmosphere within the expanding bud. 

 Active secretion of water takes place when the hydrostatic 

 pressure within the plant becomes excessive, that is when trans- 

 piration is slow and absorption considerable, a condition that is 

 most frequently attained at night, but which can be artificially 

 produced by placing a well -watered plant under a beU-jar in a 

 warm room.^ The hydathodes can therefore be regarded as 

 safety-valves which avert blocking of the intercellular spaces 



Fig. 73. — Hydathode of Lesser Celandine (Ficaria verna) in vertical section 

 (after Salisbury), ep., epithcm tissue ; St., water-pores. 



with water under the conditions just mentioned. In the case 

 of the semi-parasites they probably serve to get rid of the water 

 which is absorbed in excess from the host. The liquid exuded 

 from hydathodes is not pure water, but usually contains a very 

 small percentage of dissolved salts. In some cases, however, the 

 amount of the latter may be so considerable that they remain 

 behind as an incrustation when the water evaporates. Thus 

 in the Saxifrages {Saxifraga) a little white scale consisting of 

 carbonate of lime forms on the leaf-teeth over each of the 

 hydathodes. 



The exudation of water from the hair-like hydathodes of 

 semi-parasites and the Runner Bean is due to active secretion 

 on the part of the protoplasts of the constituent cells. As a 

 1 Cf. F. & S., p. 121. 



