240 Mr. R. Kidston on the Relationship 



upper part of the Ulodendroid scar that the leaf-scars are first 

 effaced. The continued pressure of the appendicular organ 

 against the cortex, augmented bj the increase in girth of the 

 stem, causes the bark to swell up around its base, and thus 

 the characteristic Ulodendroid depressions are formed. On 

 the upper part of the Ulodendroid scar the leaves of the 

 attached organ first obliterate all traces of the leaf-scars, and 

 finally impress their own strap-shaped outline as radiating 

 lines from the umbilicus. On the lower part of the scar cir- 

 cumstances modify the case. The space here between the 

 attached organ and the bark is much greater than in the upper 

 portion of the scar ; hence, though the attached organ effaces 

 the leaf-scars on the stem by its leaves or bracts pressing on 

 its surface, they have not sufficient power to impress their out- 

 line on the Ulodendroid scar ; thus the little " dots," which 

 mark the channels through which the foliar bundles have 

 passed are left to indicate the position of the stem-leaves. As 

 Schimper has pointed out, there cannot remain much doubt 

 that the appendicular organs result from a series of unequal 

 dichotomies, which were alternately fertile and barren, the 

 fertile probably forming deciduous cones, the barren carrying 

 on the axis of the plant. 



Of actual cases where the appendicular organ has been 

 found in situ I only know of five: — first, that mentioned by 

 Dr. Hooker* ; the second the notice given by Dr. Dawson, who 

 mentions having seen on one occasion the cones attached to 

 the stem t ; the third the specimen figured by Mr. D'Arcy 

 Thompson \ ; the fourth and fifth those shown in figures 9 and 

 11 of this communication. That the appendicular organs were 

 aerial roots, as supposed by Mr. Carruthers§, has been pointed 

 out by Dr. Williamson || and fully corroborated by the speci- 

 mens described in this paper. 



* Mem. of the Geol. Survey of Great Britain, vol. ii. part 2, p. 427. 



t Acadian Geol. 2nd ed. p. 456 (1868). 



X Trans. Edinb. Geol, Soc. vol. iii. pi. (B). 



§ Note. — The specimen on which Mr. Carruthers founded his belief 

 that the attached organs were aerial rootlets and directed downwards, 

 and which he figures on pi. sliii. fig. 5 of the Monthly Micr. Journ, for 

 March 1870, is in the collection of the British Museum (Natural History). 

 This specimen does not belong to Ulodendroxi, L. & H., but is a Halonian 

 branch of LepidopMoios. On the opposite side of the specimen to that 

 fio-ured there is a third row of tubercles ; hence it cannot be a TJloden- 

 dru7i, L. & H. Again, the leaf-scars in Leindophloios ai'e directed down- 

 wards, and the view of those he gives in fig. 6 of the same plate proves 

 most conclusively that he has been dealing with LepidopMoios, and not 

 TJlodendron. His fig. 5, then, is inverted ; so if his specimen were placed 

 in its true position; the tubercles would be directed upwards, which con- 

 forms with the ordinary Halonian branches of Lepidophloios. 



II Phil. Trans, vol. ckxii. p. 209 (1872). 



