42 LONGEVITY OF PLANTS. 



oleraeea, attain a height of one hundred and fifty to one himdied and 

 eighty feet; some Coniferse, e. ^., Pinus Lamherti, Abies Douglasi, of 

 more than two hundred feet), yet a maximum is reached there, and the 

 terminal shoot is less perfectly nourished every succeeding year, becomes 

 stunted more and more, and the tree at length dies. If we are surprised 

 at the intensity of the vegetative force of individual plants, in conse- 

 quence of which it re-appears with new, unweakened energy in every 

 bud, so must we marvel at the force committed to so simple an organ as 

 a ceU is, if we reflect what an influence it exerts upon the total economy 

 of nature, as one of the grandest of phenomena. The plant lives almost 

 solely upon inorganic substances ; its cells are chemical laboratories in 

 which these are combined into organic compounds. The plant 

 prepares in this way not only the nutriment required for its own 

 development, but also the food on which the entire animal kingdom 

 depends. But plants not only nourish animals, they maintain the air 

 in a fit state for their respiration, since their breathing process removes 

 carbonic acid from the atmosphere and replaces it by oxygen gas. In 

 all these functions the plant is thoroughly dependent upon the outer 

 world ; its food is brought to it without its own cooperation by water and 

 air ; its respiration takes place without activity of its own, through a 

 penetration of its substance by gases with which it is in contact, in 

 consequence of a physical law ; not even does its internal circulation of 

 juices depend on a, mechanical activity of a circulating system ; thus 

 every necessity for motion is removed. It is true we here and there 

 meet with movements in this or that organ, hut these, occurring isolated 

 in the vegetable kingdom, are also altogether of subordinate kind in the 

 individual plant." 



The stem of a plant consists, then, of the following paxts, viz. : 



1. Wood, the oldest of which is heart- wood, and the newest 

 alburnum ; this is the substance through which sap ascends : 



2. Bark, the external coating, down the liber or inner face of 

 which sap descends : 3. Pith, a central portion of the horizontal 

 system : and, 4. Medullary Bays, serving to connect the bark 

 with the pith, to hold all the parts together, and to maintain a 

 communication between the centre and the circumference of a 

 stem. The stems of all plants have these four parts more or 

 less evident. They are most visible in European trees or 

 shrubs, in any of which they can be distinctly observed ; they 

 are least apparent in annual and herbaceous plants, because 

 their lines of separation are not defined, aU the four parts 

 adhering to each other so firmly as to render it difficult to 



