LONGEVITY OF PLANTS. 41 



longer period. It is, howeYer, not probable that Exogenous trees have 

 a power of indefinite life. Upon this subject the foUowing remarks by 

 Professor Mohl are among the best which have been made : — 



"The peculiarity of their origanisation, and the unlimited power of 

 growth of plants, offer many difficulties to the definition of the duration 

 of plants, and have given rise to many incorrect theories. Every 

 individual cell, and evesry individual organ, has a determinate end to 

 its life ; but the entire plant has not, since the individual shoots run 

 through their periods of development quite independently, and only share 

 in the weakness of age of the older organs when these are no longer 

 able to convey to the young shoots the needful amount of nourishment, 

 in which case the latter do not die from deficiency of vital energy, but 

 are starved. It therefore depends wholly upon the mode of growth of a 

 plant whether this occurs or not. When a plant possesses a thallus 

 spreading horizontally by the growth of its circumference, it can 

 annually extend itself into a larger circle, after the old parts in the 

 centre have been long decayed, as is seen in old specimens of orustaceous 

 Lichens, in the fairy rings caused by Fungi, &c. In like manner when 

 a higher plant has a creeping stem, and possesses the power of sending 

 out lateral roots near the vegetating points, and in this way conveys 

 nourishment directly to the young terminal shoots, the latter are wholly 

 independent of the death of the older parts of the stem and of the 

 primary roots, and there exists no internal cause for death in such a 

 plant. It is truly a different plant every new year and vegetates in a 

 new place, but there is no definite boundary between it and its prede- 

 cessors ; such a plant is Uko a wave rolling over the surface of a sheet 

 of water; it is every moment another and yet always the same. 

 Thousands of inconspicuous plants, of Mosses, Grasses, Rushes, &c., 

 have vegetated in this manner upon peat bogs and similar localities 

 perhaps for thousEtnds of years. Plants with upright stems are placed 

 in much more unfavourable circumstances. It has been declared of 

 these alsOj and particularly of the Dicotyledonous trees (De Candolle, 

 Physiohgie V4gitale, ii. 984), that they have no internal cause for 

 death, but I believe incorrectly. Examples of very old trees, such as 

 De CandoUe coUeoted (e. g., Taxus 3000, Adansonia 5000, Taxodium 

 6000 years old, &c.), only prove, naturally, that death occurs at a very 

 late period in many plants placed in favourable circumstances, but not 

 that it does not necessarily happen. To me there appears to exist in all 

 trees, whether they belong to the Dicotyledons (Exogens), or, like the 

 Palms,, to the Monocotyledons (Endogens), an internal cause which 

 must produce death in time — namely, the increasing difficulty of 

 conveying the necessary quantity of nourishment to the vegetating 

 point, resulting from the elongation of the trunk from yeax to year. 

 Even when the force which carries the sap up, suffices to raise it to two 

 hundred feet or more (many Pabns, as Ceroxylon andicola, Areca 



