80 AFFERENT AND EFFERENT VESSELS [CH. 



effect, as the silk of an umbrella is extended on its sup- 

 porting framework of steel ribs ; and that the petiole may 

 be compared to the handle and stick of the umbrella 

 which lifts the expanded tissue up into the light and air. 

 It has also been shown that the supporting network of 

 fibrous structures is accompanied by a series of afferent 

 pipes which conduct water from the roots to every part of 

 the leaf, and which we compared to the water-supply of a 

 town, and by a similar series of efferent pipes which 

 convey other fluids in the reverse direction. This latter 

 system reminds us in its distribution of the drainage- 

 system of the town, only, as was pointed out, the materials 

 gathered up into it from all parts of the leaf, and con- 

 veyed by it into the plant are very different from the 

 waste-products poured into the sewers : they are, on the 

 contrary, the nutritious food-materials on which the rest 

 of the living parts of the plant are to be fed. 



Nevertheless, in its broad features the analogy holds, 

 and we may conclude that the afferent system of pipes 

 conveying the water brought up from the roots, and con- 

 taining traces of salts such as chlorides, sulphates, phos- 

 phates and nitrates of magnesium, calcium, sodium arid 

 potassium, which must inevitably be dissolved in such soil- 

 water, as well as the efferent system of pipes conveying 

 very different solutions away from the leaf to places where 

 they are wanted for nutrition in the plant, simply accom- 

 pany the supporting framework as a matter of economy 

 and adaptation. 



The network of pipes and fibres referred to is easily 

 rendered conspicuous by any of the processes used for 

 obtaining the so-called skeleton of a leaf, the commonest 

 of which is the natural one of allowing the leaf to lie in 

 stagnant water until the process of rotting has gone so 

 far that the softer tissues of the leaf are reduced to pulp 



