ORGANIC METAMORPHOSIS OF LEAVES. 153 



after the vegetative powers of the main stem and its 

 branches have been exhausted by the development of 

 branchlets and twigs ; for, as a general rule, any thing that 

 checks vegetation increases the tendency to reproduction. 



It has been proved, in Chapter VI, that the vegetative 

 powers of the whole tree, in common with those of each 

 leaf, shoot, and branch, have a period of acceleration and 

 retardation like the rising and the falling of a wave. It 

 is at that period, emphatically called the " change of 

 life," when these powers have just passed their culminat- 

 ing point, when the vegetative vigor of the tree begins 

 gradually to decline, that we notice a peculiar alteration 

 in its habits and structure, for the sap of the tree is no 

 longer totally expended in the formation of new shoots 

 and leaves, but partly in the production of flowers and 

 fruit. 



In herbaceous plants, no flowers manifest themselves 

 until the organization has first acquired the degree of 

 strength which is necessary to produce them, by passing 

 through a certain series of vegetative stages. The time 

 required for this purpose depends upon the peculiar consti- 

 tution of the plant: in some, a few days or weeks is all 

 that is necessary to form a vegetative foundation for the 

 flowers; the stage of puberty is reached, and the plant 

 unfolds its flowers, forms its seed, and then dies. 



This is the case with some of our native annuals, such as 

 the Whitlow Grass (Draba verna), which covers the ground 

 with a profusion of small white flowers, through the 

 months of March and April. I have always felt an especial 

 interest in this plant, on account of the beauty and sim- 

 plicity of its organization, and the brevity of its life. The 

 radical leaves, which are disposed about the scape, or 

 flower- stem, in a stellate manner, make quite a graceful 

 appearance on the ground ; the scape itself rises from 

 their centre to a height varying from one to three inches, 

 and supports from eight to twelve small white flowers, 

 which form little, smooth, ovate-lanceolate seed-pods. 

 This lowly plant lives about two or three weeks ; it then 

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