May 23, 



!■] 



SCIENCE. 



757 



leads to this conclusion: " Unlike what 

 occurs amongst the living Lycopods, 

 amongst the Carboniferous Lepidodendra 

 we find as we descend from the uppermost 

 and youngest shoots, that there is a regular 

 progressive enlargement of the branches 

 below each succeeding dichotomy; * * * 

 and these enlargements are accompanied 

 by a similar though less conspicuous en- 

 largement of the cylinder of the primary 

 xylem, and also in the number of its com- 

 ponent tracheids." 



Prof. Williamson's examinations relate 

 in particular to seven species of Lepidoden- 

 di'on.* Of these L. Selaginoides differs from 

 other studied Lepidodendra in having the 

 tracheids of the primary xylem, which are 

 crowded at the outer periphery, more open 

 and fewer in approaching the center of the 

 system, where they often mingle with a pe- 

 culiar barred parenchyma that occupies the 

 place of the medullary cells in other species. 

 In one specimen of this species the primary 

 xylem has reached a diameter of nearly 

 S mm., the cortical diameter being nearly 

 17 mm. before a small crescent of secondary 

 xylem is discerned. At a more advanced 

 stage the diameter of the primary xylem 

 cylinder is 6 mm., the secondary xylem 

 14.5 mm., while the cortical is 92 mm. 



Of L. hrevifolium, which is remarkable for 

 its frequent dichotomies, of both the equal 

 and unequal types. Prof. Williamson ob- 

 tained a section, below a dichotomy, in 

 which a secondary xylem of a maximum 

 thickness of 5 mm., invested the two 

 tracheal crescents of primary xylem, the 

 secondary xylem tissue being seen to grow 

 around the horns of each of the primary 

 * We can refer to "but a few of the author's observa- 

 tions. Those who wish further data will find such 

 tabulated in the present paper and illustrated in the 

 magnificent series of memoirs ' On the Organization 

 of the Fossil Plants of the Coal Measures,' pub- 

 lished by Prof. Williamson during the last twenty- 

 five years in the Transactions of the Eoyal Society of 

 London. 



xylem crescents and to push its way into 

 the interior of their contained meduUse. 



Exceptionally favorable conditions of pre- 

 servation have made it possible to trace the 

 development of the tissue in L. Wunschianum 

 from the youngest twigs down to stems sis 

 feet in circumference. In this plant speci- 

 mens in which the primary xylem is 4 mm. 

 in diameter show no medulla, though on 

 reaching a diameter of 5.5 mm. a medulla 

 nearly 2.5 mm. in diameter appears. But 

 the remarkable fact that the smallest stem 

 in which a trace of secondary xylem was 

 found showed the diameter of the cortex, 

 primary xylem and of the medulla to be 

 23 cm., 36.5 mm. and 24 mm., respectively, 

 while the very thin ring of secondary xylem 

 is but 4 mm. thick on one side and 1 mm. 

 thick on the other, demonstrates that the 

 branches of this species attained a relatively 

 large size before the growth of secondary 

 xylem began. 



In L. Harcourtii, the sLudy of a section of 

 which led Brongniart astray and began the 

 conflict between the English and the French 

 paleobotanists, the author elucidates several 

 minor disputed points. It is noteworthy 

 that no exogenous or secondary growth has 

 yet been found by any of the investigators of 

 this species, for the possible reason, as Wil- 

 liamson suggests, that the secondary xylem 

 does not appear until a stage more ad- 

 vanced than that represented in any speci- 

 mens yet examined. 



Two of the sections of L. fuUginosum give 

 the following diameters: 1st — cortex, 19 

 mm.; primary xylem, 3.5 mm.; medulla, 2 

 mm. 2d— cortex, 60 mm.; primary xylem, 

 7 mm.; medulla, 6 mm. At an advanced 

 stage of growth, among the radial lines or 

 cells of the innermost cortex, are found par- 

 allel lines of true tracheids, 'rudimentary re- 

 presentatives of the secondary xylem zone.' 

 These cells pursue an irregular course longi- 

 tudinally, and are unequallj^ distributed in 

 the cortical ring in which they occur. 



