134 THE STEM. 



ing exogenous stem, the part above the ligature swells ; that below 

 does not. Every one may have observed the distortions that twin- 

 ing stems thus accidentally produce upon woody exogenous trunks. 

 On examination, the woody fibres are found to be arrested at the 

 upper margin of the ligature, and thrown into curved and knotted 

 forms ; or, where the ligature is spiral, the descending fibres fol- 

 low the course of the obstruction. (2.) When we girdle an exoge- 

 nous stem, by removing a ring of bark so as completely to ex- 

 pose the surface of the wood, the part above the ring enlarges in 

 the same manner ; that below does not, except by the granulation 

 of cellular tissue, until the incision is healed. (3.) In a graft, the 

 descending wood of the scion may often be seen to be quite dis- 

 tinct from the stock ; the latter sometimes dies while the scion 

 continues to grow. (4.) In many cases the fibres of wood are 

 found to curve abruptly round a projection, gradually resuming 

 their perpendicular direction below. Sometimes they take a very 

 sinuous course, when there is no obstruction or evident cause 

 of disturbance ; the fibres of adjacent layers even crossing each 

 other at right angles, showing an entire independence of the ante- 

 cedent layer in their growth. (5.) The wood of the roots is ad- 

 mitted to grow in a descending direction. But it is continuous 

 with that of the stem ; and its first layer, the extension of the 

 wood of the radicle into the primary root, agrees in composition 

 with the wood of the succeeding layers in the stem, having no 

 spiral vessels, but only ducts. 



227. We have seen (148) that lateral buds develope into branch- 

 es, just as the original embryo developed into the primary stem. 

 Now the original embryo, or primary bud (144), not only grew up- 

 wards to form the stem, but downwards to form the root. Buds 

 grow upwards into branches ; have they aught corresponding to 

 the downward growth which in the original stem is represented by 

 the roots ? The answer is furnished by those buds which may be 

 made to grow independently of the parent stem ; such, for in- 

 stance, as the bulblets of the Tiger Lily (Fig. 143), which are 

 merely axillary buds with fleshy scales, and which, when they fall 

 to the ground, or even while yet in their native situation, emit 

 rootlets from their base, whose downward growth is the counterpart 

 of the upward growth of the stem to which the bud gives rise. 

 The same evidence is furnished by those ordinary buds which nat- 

 urally grow in union with the parent, but which the gardener 



