156 TEMPSKYA. 



cylinder. "We must remember, however, that under natural con- 

 ditions of growth the case may be otherwise. Be that as it may, 

 my examination of the lower parts of cultivated Dicksonia antarctica 

 stems does not afford any strong support to Velenovsky's view. 



Could the thick enveloping mass of roots readily become separated 

 from the central vascular axis of a tree-fern stem, and thus account 

 for the occurrence in a fossil state of thick bundles of adventitious 

 roots without the central stem axis? We know how it has fre- 

 quently happened in the case of Lepidodendra stems from the 

 Coal-Measures that the central vascular axis has been completely 

 separated from the outer cortical tissues, and the latter have thus 

 been compressed together, forming an apparently complete 

 specimen. This separation in Lepidodendron is easily explained 

 by the two concentric lines of weakness which exist in the meris- 

 tematic layers, the cambium of the central cylinder, and the 

 meristematic zone in the outer cortex. In Dicksonia and other 

 tree-ferns we have no such zones of delicate cells, along which the 

 tearing apart of tissues might readily take place ; on the contrary, 

 we have the central vascular tissue with numerous spirally placed 

 petioles bound together by the plexus of roots, and it is not easy to 

 understand how any separation could be effected during fossilization. 

 In Museum specimens of Dicksonia antarctica stems, I am unable 

 to detect any tendency to a clean separation of the surrounding 

 roots from the central axis. 



Again, it does not seem probable that the central axis would be 

 disorganized, and the roots remain as mineralized structures; the 

 thick resistant bands of strengthening tissue which accompany the 

 vascular plates would be far more likely to withstand weathering 

 influences than the smaller root structures. 



Yelenovsky suggests that the roots of Tempskya probably drew 

 up from the soil the mineralizing solutions, which eventually 

 replaced their organic cell-walls. It is perhaps conceivable that 

 the roots on the base of the stem may have taken up calcareous 

 or siliceous solutions, and that the central vascular axis did not 

 offer any such convenient path for their ascent ; if this were so 

 the axis would gradually decay, and subsequent compression of the 

 root envelope might close up the vacant space and leave no signs in 

 the mineralized mass of any axial structure. This, however, is 

 mere speculation, and probably of little or no value in the solution 

 of this difficult question. 



