28 



FUNCTIONS OF PLANTS, CONSIDERED 



arises from the base of the seed-leaves, and is m general in very small 

 quantity ; but, as soon as the proper leaves appear, the quantity of woody 

 matter formed is considerable, even during the first growing- season. 



When this woody matter first penetrates the cellular tissue of the infant 

 stem, it forms a little circle within its circumference, and thus separates the 

 interior of the stem into two parts. These parts are, the bark or exterior 

 portion, and the pith or central part ; and between these, at least in all 

 exogens, there is a third portion, which constitutes the wood. 



108. Organically, the stem may be said to consist of two parts : the 

 cellular tissue, which is not, from its nature, capable of increasing by growth 

 more in one direction than in another ; and the woody fibres, which are trans- 

 mitted from the leaves through the stem, and down into the roots. In 

 speaking of the construction of stems, the cellular tissue in them is called 

 the horizontal system ; and the woody fibres, as they increase longitudinally 

 by the addition of new fibres or tubes having the same lengthened direction 

 as themselves, are called the perpendicular system. 



109. Wood.^ in exogenous plants^ consists chiefly of the perpendicular sys- 

 tem, while the pith in the centre of the stem, and the bark on its circum- 

 ference, are chiefly formed of the horizontal system. The bark communi- 

 cates with the pith by the continuation of the cellular tissue through the 

 woody fibres ; and the cellular tissue, seen among these woody fibres in the 

 section of a tree made smooth by the plane, is called the medullary rays, 

 from the pith in plants being supposed analogous to the medulla of animals. 

 Hence the section of the trunk of a tree has been compared to a piece of 

 cloth ; the horizontal system, or medullary rays, representing the woof. 

 and the woody system the warp. 



110. When a stem is injured by the removal of a portion of the bark of 

 such a depth as to reach the wood, the wound is healed over ; first, by the 

 cellular matter oozing out of the last-formed wood, and granulating on the 

 surface ; and secondly, by this ^.ellular matter being penetrated by the fibres 

 of the perpendicular system. Rings of bark are frequently cut from the 

 stems of trees for the purpose of checking the returning sap, either to cause 

 the tree to produce blossoms, or for the purpose of inducing the stem or 

 branch to throw out roots along the upper edge of the part from which the 

 bark has been taken. The immediate efi^ect of the process is the protrusion 

 of granulated matter, or cellular tissue, along both sides of the wound, but 

 especially on the upper side. Now, if the wound be surrounded with a 

 quantity of moss, tied firmly on, and kept moist, the perpendicular system, 

 or ligneous fibre, will penetrate through the granulated matter, and become 

 roots ; while no roots whatever will be protruded from the granulated matter 

 on the under side of the wound ; thus proving, firstly, the truth of the 

 theory of the perpendicular system ; and secondly, that roots, in growing- 

 plants, are formed by the protrusion of woody fibre through cellular matter. 

 The first process of nature, when a cutting is formed and planted in the soil, 

 is to protrude cellular matter round the edges of the section of its lower 

 extremity ; this protruded matter, or callosity, as it is termed by cultivators, 

 sometimes remains for several months before it undergoes any change ; but 

 ultimately, if the cutting succeeds, the perpendicular system passes through 

 it and appears in the form of roots, and the cutting is established as a plant. 

 If a cutting be planted in the soil in an inverted position, though the 

 portion in the soil be cut and nrepared as in cuttings treated in the usual 



