of the Stem of the Phanerogamia. 405 



in the buds ; but in the adventitious buds of Begonia phyllo- 

 manica it is beyond all doubt that the vascular bundles are de- 

 developed quite independently of the cambium-layer of the stem. 

 These buds originate in the outermost layer of the rind, and are 

 separated from the cambium-layer by the entire, perfectly un- 

 altered cellular tissue of the rind — at times, when they have 

 already formed air-bearing spiral vessels in their axes and leaves. 

 The dogma that vascular bundles can only originate by the 

 ramification of other vascular bundles, is therefore decidedly 

 false. 



Another axiom set forth by Schacht is, that only those vas- 

 cular bundles whose cambium-layers coincide with the general 

 cambium-layer of the stem can become thickened in the radial 

 direction. Schacht on this account applies to the cambium- 

 layer the name of thickening-ring (Verdickungsring) , and attri- 

 butes the incapacity of the Monocotyledons to increase in thick- 

 ness to the want of this coincidence. 



This idea also I must declare to be erroneous. I mentioned above 

 that in the stems of Cucurbita, Cucumis, &c, and Basella, the 

 vascular bundles lie inside the medullary parenchyma ; in young 

 stems there is no trace of a cambium-ring connecting them; 

 but this does not prevent the vascular bundles from growing 

 very considerably in thickness, in the same manner as in other 

 Dicotyledons. This would cause the rupture of the cellular 

 tissue, comparable to medullary rays, lying between the vascular 

 bundles, were it not that a secondary cambium-layer is formed, 

 in consequence of this increasing thickness of the vascular bun- 

 dles, by the division of the parenchymatous medullary cells 

 between the latter, which renders possible the elongation out- 

 wards of the medullary rays. Here it is very evident that the 

 vessels do not increase in thickness because they lie in a cam- 

 bium-layer, but, on the contrary, a cambium-layer is subse- 

 quently formed on account of the thickening of the bundles. 



A secondary cambium-layer may be formed not only in a 

 direction parallel to the surface of the stem, but a similar pheno- 

 menon may occur in a transverse direction across the whole thick- 

 ness of the stem, and then give rise to the formation of vascular 

 bundles, which subsequently apply themselves to the bundles 

 already formed from the cambial sheath of the stem. This 

 occurs in the nodes containing a vascular plexus both of the 

 Monocotyledons (for instance Grasses) and Dicotyledons (e. g. 

 Ricinus). A longitudinal section through the terminal bud of 

 these plants shows that these nodes originate at a relatively 

 late period ; for no indication of them can be found in the 

 internal cellular tissue of the axis of the bud, and the me- 

 dullary cellular tissue of the latter consists of uniform, re- 

 draw, k Mag. N. Hist. Ser. 3. Vol. ii. 28 



