Vol. 63.] XEROPHYTIC CHAEACTEES OF COAL-PLANTS. 289 



events, they would be practically useless to aerate trees 60 to 70 or 

 more feet in height. In certain mangroves the hollow pneumato- 

 phores are as large as, and actually used for, beehives. 



Moreover, the presence of a three to four-celled zone of very thick- 

 walled cells is not characteristic of the roots of water-plants. If, for 

 example, the root of the common flag (Iris Pseud-acorus), growing in 

 water, be compared with one of the same plant growing in a garden, 

 the latter has the cells of the endoderm consisting of enormously- 

 thickened walls, whereas in the former they are all very thin- 

 walled indeed. Had the Stigmarian rootlet been growing in water, 

 the above-mentioned zone would have decreased or disappeared. 

 The very presence of the lateral tracheids is a proof of the difficulty 

 of securing water. 



Dr. Weiss concludes his paper thus : — 



' The existing type of aquatic plant with a monarch root, Isoetes, has, as a 

 submerged plant, a much smaller requirement for root-absorption, and is 

 apparently able to dispense with this additional method of conduction shown 

 to be characteristic of the various types of Stigmarian rootlets.' 



The reference seems obvious, that the latter require it because 

 they were not aquatic, but xerophytes. 



Dr. Scott gives a figure, 123 (op. cit. p. 367), of Ptyclioxylon 

 (Cycadoxylece), as well as one of Lyginodendr on, 122 (p. 363), show- 

 ing very anomalous characters in the wood. In the former stem 

 the leaf-gaps fail to be filled up by the closing-in of the xylem 

 cylinder-bundles, while these have inverted additional strands. It 

 is a common feature of shrubby desert-plants to have various kinds 

 of anomalies in their stems. It appears to be in part due to the 

 feebleness of the development of foliage, as seen in their reduced 

 size (partly compensated for by the increase in palisade-tissue), so 

 that the stem is unable to make the normal amount of wood and 

 proper arrangement of bundles, occurring in stems of mesophytic 

 trees. I have described some such anomalies elsewhere. 1 



With regard to the anatomical structure of the leaves, transverse 

 sections (in Dr. Scott's fig. 139, op. cit. p. 423) exhibit strong 

 xerophytic characters, such as a thick cuticle, a triple series of 

 palisade-cells below the upper epidermis, and two beneath the 

 lower one. There is also much sclerotic tissue, with a total absence 

 of lacunas, except such as is normally present in the lax mesophyll- 

 tissue. 



The foregoing references are quite sufficient to show that, while 

 the Equisetales were decidedly hygrophytic, most of the rest of the 

 Palaeozoic plants — like their modern representatives in Lycopodium 

 and all Gymnosperms — were markedly xerophytic. This implies 

 that the majority of the coal-plants did not grow in swamps. 



Perhaps the nearest approach to the true conditions may be seen 



1 ' The Origin of Plant-Structures by Self-Adaptation to Environment ' 1895, 

 p. 73. 



Q.J.G.S. No. 251. x 



