COLLEGE OF 

 AGRICULTURE 



Berkeley. Cat. 



- .I i ^ 



[333] SCIENTIFIC MANUAL. 47 



first, the embryo, which, by a continuation of the same 

 process, develops into the plant or tree. New cells can 

 only be formed from the organizable matter already assim- 

 ilated in the interior of older cells. The enlargement of 

 the stems of exogenous plants is occasioned by the appro- 

 priation of organizable matter, which has been prepared in 

 the foliage and passed downward, in consequence of its 

 increased density, by osmose, through successive cells in 

 the inner bark, until it is appropriated to the formation of 

 new cells, either in the production of new rings of cam- 

 bium, or sap-wood, or of bark tissue 



That stems increase in size by the appropriation of pre- 

 pared sap from above, is shown by several familiar facts 

 which have fallen under the observation of most farmers. 



When a wire is left tightly bound around a stem, it will 

 enlarge more above the wire than below, until, when by 

 continued growth, the stem increases the binding force of 

 the wire, so that enlargement below entirely ceases. 



The same phenomenon is seen when a vine entwined 

 about a tree so tightens its grasp as to. arrest the descent 

 of the prepared food through the inner bark, an enlarge- 

 ment of the stem takes place above the circle of the vine's 



embrace. 



Branching trees increase in size rapidly towards their 

 base, and in proportion to the surface of the foliage ex- 

 posed by the successive branches. The trunks of palms, 

 which have leaves only at the top, increase in size but little 

 towards the base not more than may be accounted for by 



priority of formation. 



The same is true, to a great extent, of forest pines, which 

 lose their lower limbs as they increase in height. 



The growth of roots is controlled by the same general 

 laws which determine that of stems, as far as the source of 

 prepared food is concerned. Their increase in length, as 

 before remarked, is confined to accretion by new cell form- 

 ation, and never, as in stems, by elongation. 



