STRUCTURE OF PLANTS. 197 



stems and flowers of many, and the whole of a very 

 considerable number, and these last, though called 

 annuals, do not exist for that length of time, many of 

 them performing the whole of their functions in a few 

 weeks ; but we are in the habit of calling all flowers 

 annuals that die, roots and all, in the ordinary course 

 of nature, after they have flowered once, and that do 

 not take more than one revolution of the year from 

 the time that the seeds germinate. Many of the bulbs 

 are biennials in respect of the particular bulb, while 

 they are perennial in the whole plant, by the succes- 

 sive production of new bulbs, as is the case with lilies ; 

 others have no reproduction of the bulb, but die 

 wholly when the seed is ripened, as is the case with 

 the parsnip and carrot. But though these last exist 

 through a part of two years, as counted from the 

 winter solstice, and though the leaves of the first year 

 die down, in part or in whole, during the winter, they 

 are only annuals in duration, and in a state of nature 

 sow their seeds every summer, and die when that is 

 accomplished. Trees are the most interesting of the 

 perennial plants, and the general process is, perhaps, 

 best seen in them, as they exhibit all the stages of it. 

 A farther variety is produced by some plants that are 

 continually dying at one part of the root, and repro- 

 duced at another ; but even in these, the general 

 process is still the same. 



The stem of a plant may always be considered as 

 made up of three parts, even in its earliest and most 

 simple state. These are, the pith in the centre, which 

 in perennials diminishes as the plant gets old, and in 

 annuals is often absorbed in the course of growing, or 



