~.'>'S PHYSIOLOGICAL BOTANY. PART II. 



attached is immersed, then the water is readily ab- 

 sorbed and penetrates into the misseltoe itself. 



(235.) Duration of Life. Some plants exist only 

 for a few days or weeks, others for about a twelve- 

 month or two years, and others again for a very length- 

 ened period. Some when they have once flowered 

 and perfected their seeds immediately die ; and these 

 in consequence are termed " Monocarpeans." Others 

 annually produce a fresh crop of seeds, and are termed 

 " Polycarpeans." The difference between them is more 

 apparent than real ; for although in the ordinary course 

 of things the Monocarpeans soon die, the natural 

 period of their existence may be considerably extended 

 beyond the usual period, by merely preventing the form- 

 ation or development of their seed. This shows us 

 that it was the effort of the plant to form seed which 

 checked the functions of nutrition, and not that the 

 period of its existence was necessarily so limited as 

 its early death would seem to indicate. Some plants 

 which are annuals in our stoves are perennials in their 

 native country. The American aloe (Agave americann) 

 is a striking example of a plant, the ordinary period 

 of whose existence may be very considerably extended 

 by preventing its flowers- from developing. In its na- 

 tive climate it comes into blossom when four or five 

 years old, and afterwards dies ; but in our greenhouses 

 it continues to vegetate for fifty or a hundred years 

 without showing any symptoms of putting forth its 

 flowers. If then we make abstraction of those checks 

 which are given to the vital function by the process of 

 fructification, and which do not appear formidable in 

 any degree to the life of perennial species, we might 

 imagine it possible for plants to continue vegetating for 

 a much longer period than they naturally would ; and 

 that the life of some might be extended indefinitely, 

 provided the external or accidental causes which tend to 

 produce decay and death were continually removed. By 

 this we mean, that certain plants never die from the 

 effects of old age in the same sense in which we apply 



