98 MR. T. HICK ON A NEW FOSSIL PLANT 



stem under consideration. The scantiness of the material pre- 

 vents the formation of a definite conclusion as to the origin of 

 the radicular tissues, hut it proves indisputably that small 

 collateral bundles become separated from the vascular-bundle 

 ring, and that while still near the ring they are imbedded in the 

 pericycle and have no special parenchymatous envelope of their 

 own. On the assumption — which is surely not unreasonable 

 that the vascular bundles further from the rinjr have a similar 

 origin, it will follow that as the distance from the bundle-ring 

 increases, the isolated bundles become surrounded by a special 

 envelope of parenchyma, which may be differentiated into 

 distinct layers. So far as appearances go, then, they suggest that 

 these root-like structures originated by the separation of 

 collateral bundles from the bundle-ring, accompanied by an 

 active multiplication of the cells of the pericycle. At a further 

 distance from the centre, this activity is still continued and a 

 zone of parenchyma round each bundle becomes more or less 

 clearly distinguishable from the surrounding parenchyma. 

 There is, however, no break in the organic continuity, and even 

 when the special parenchyma is differentiated into concentric 

 and radiating layers, the latter pass over, in many cases, without 

 interruption into the surrounding parenchyma. Whether the 

 formation of periderm and the splitting of the tissues are the 

 last stages by which these structures ultimately become free we 

 cannot say ; but such a view would be consistent with all that 

 has been said, and would complete the story suggested by the 

 structure of the radicular tissues. 



It scarcely needs to be said that such a mode of origin for 

 structures which are probably roots differs materially from 

 anything that is known to occur in living plants. On this, if 

 on no other account, then, it can only be regarded as a specula- 

 tive effort to explain the structure and arrangement of the 

 tissues involved, and as such it is here put forward. Neverthe- 

 less, it is to be noted that in describing the mode of branching 

 met with in the roots of Lycopodium clavatum, Van Tieghem 

 shows that here, too, branching is preceded by a division of the 

 vascular strand. Not only so, but he states explicitly that when 

 the vascular strand divides unequally, the smaller portion runs 

 for a long time in the cortex of the primitive trunk, nearly 

 parallel to the larger portion, before it acquires its own special 



* " Memoir sur la Eacine," Ann. d. Sci. nat, Botanique, serie v. tome xiii. 1870. 



