298 VEGETABLE ORGANOGRAPHY. 



their apex is the first part which appears, and they are 

 elevated in issuing from the bulb as if they were pushed 

 up from below. Upon a leaf of this kind, half developed, 

 I marked points at equal distances ; these marks 

 remained at the same distance at which I placed them, 

 but the lowest was found further from the base from 

 the development of the part situated below it, and before 

 buried in the bulb ; thus, while the branches of the year 

 elongate throughout their whole length, and the roots 

 by their extremities alone, these kinds of leaves or 

 petioles do so by their base. 



Is it the same with ordinary petioles, and nerves, which 

 are only the divisions of petioles ? This is what I am. 

 inclined to believe, but I cannot as yet affirm it, for 

 want of conclusive experiments. 



The growth in breadth is essentially owing to the 

 elongation of the lateral fibres in all plants with ramify- 

 ing or diverging nerves, and to the development of the 

 intermediate parenchyma. As for the enlargement of 

 those with parallel or conveying nerves, it is generally 

 slight, and only appears to be owing to the development 

 of intermediate cellular tissue ; it is also remarked that 

 the breadth of leaves is much less subject to variations 

 in these than in the preceding. 



The growth of leaves, both in length and breadth, 

 attains its limit generally very rapidly ; then the leaf 

 performs its functions for some time, and enjoys the 

 plenitude of its existence ; but by degrees, from its ex- 

 haling perfectly pure, and, as it were, distilled water, 

 and retaining in its tissue earthy matter which the sap 

 has carried there, the vessels harden, and the exhaling 

 pores are obstructed : this term arrives generally the 

 more rapidly as the evaporation is more active. We see 

 the leaves of herbaceous plants, or of trees which evapo- 

 rate much, fall off" before the end of the year which has 



