154 ON THE ORIGIN 



the extension of parts previously organised, and in a great number of 

 cases, which must be familiar to every person's observation, raises the 

 cotyledons out of the mould in which the seed is placed to vegetate. 

 The mode of growth of the radicle is therefore similar to that of the sub- 

 stance which occupies the spaces between the buds near the point of the 

 succulent annual shoot, and totally different from that of the proper root 

 of the plant, which I conceive to come first into existence during the ger- 

 mination of the seed, and to spring from the point of what is called the 

 radicle. At this period, neither the radicle nor cotyledons contain any 

 alburnum, and therefore the first root cannot originate from that sub- 

 stance ; but the cortical vessels are then filled up with sap, and appa- 

 rently in full action, and through these the sap appears to descend which 

 gives existence to the true root. 



When first emitted, the root consists only of a cellular substance, 

 similar to that of the bark of other parts of the future tree ; and within 

 this the cortical vessels are subsequently generated in a circle, inclosing 

 within it a small portion of the cellular substance, which forms the pith 

 or medulla of the root. The cortical vessels soon enter on their office of 

 generating alburnous matter ; and a transverse section of the root then 

 shows the alburnum arranged in the form of wedges round the medulla, 

 as it is subsequently deposited on the central vessels of the succulent 

 annual shoot, and on the surface of the alburnum of the stems and 

 branches of older trees *. 



If a leaf-stalk be deeply wounded, a cellular substance, similar to that 

 of the bark and young root, is protruded from the upper lip of the wound, 

 but never from the lower ; and the leaf-stalks of many plants possess the 

 power of emitting roots, which power cannot have resided in alburnum, 

 for the leaf-stalk does not contain any ; but vessels, similar to those of 

 the bark and radicle, abound in it, and apparently convey the returning 

 sap ; and from these vessels, or perhaps more properly from the fluid 

 they convey, the roots emitted by the leaf-stalk derive their existence ( . 



If a portion of the bark of a vine, or other tree, which readily emits 

 roots, be taken off in a circle extending round its stem, so as to 

 intercept entirely the passage of any fluid through the bark, and any 

 body which contains much moisture be applied, numerous roots will soon 

 be emitted into it immediately above the decorticated space, but never 

 immediately beneath it : and when the alburnum in the decorticated 

 spaces has become lifeless to a considerable depth, buds are usually pro- 

 truded beneath, but never immediately above it, apparently owing to the 

 I.* See above, No. II. Plate 4. f Ibid. No. II. 



