148 Vegetable Physiology. 



annual set of woody bundles, proceeding from the leaves, 

 passes down through the soft interior of the stem, and after- 

 wards turns outwards, and interlaces itself with those pre- 

 viously formed. The cellular portion of the stem, which in 

 exogens was separated by the wood into the pith and bark, 

 here remains intermingled with the wood during the whole 

 life of the plant. Each woody bundle contains ducts 

 and spiral vessels, besides woody fibre, arranged so that 

 the spiral vessels are on the side next the centre, and pro- 

 tected by the woody fibre on the exterior. From this pecu- 

 liar structure endogenous stems increase little in diameter, the 

 hardness of the outside preventing their enlargement. On 

 this account too, their age is generally limited, since the exte- 

 rior vessels become so compressed as to admit no further pas- 

 sage of fluid. A remedy for this is sometimes provided by 

 Nature, in the splitting of the hard envelope which allows the 

 interior to dilate ; and this has been imitated by splitting the 

 outside of a Palm with a hatchet, when its vigor was restored. 

 The same cause which limits the age of endogens prevents 

 them from being injured like exogens, by ligatures around their 

 stems. If a cord be tied tightly around the trunk of a young 

 cherry tree, it will prevent the sap from descending to the 

 roots, and a protuberance will be formed above the cord, which 

 will increase by the superfluity of nourishment afforded it, so 

 as to bury the ligature beneath it. The same effect is pro- 

 duced by the embrace of several kinds of climbing plants, 

 though as they generally wind in a spiral direction, the descent 

 of the sap is not so completely prevented, but the woody mat- 

 ter accumulates above the whole line of the spiral, so that 

 when the creeper is removed, a deep indentation is seen pass- 

 ing round the branch or stem from one end to the other. This 

 never takes place in endogens. 



The twining stems alluded to appear to have a peculiar ten- 

 dency to turn to one side, which constantly operates in con- 

 nexion with the general tendency of all stems to grow upright 

 The direction of this turning tendency is usually contrary to 

 that in which the sun appears to move, but sometimes the 

 same with it. The common direction may be observed in 

 the case of most plants of the Pea tribe, the Convolvulus, and 



