92 
PROFESSOR W. 0. WILLIAMSON OX THE ORGANISATION 
what occurs amongst living plants that the sceptics are unable to accept my account 
of it. My late friend Professor de Bary, when his students occasionally declared that 
certain suggested explanations were “ impossible,” usually replied, “ in scientific 
research nothing is ‘impossible.’ ” I utilise this philosophic reply, and commend it to 
some of my new opponents. 
I now advance further and proceed to show tliat the process of medullary develop¬ 
ment which, as demonstrated in my Memoir XVI., occurs so commonly amongst the 
Lepidodendroid stems and branches, takes place, if possible, in even a more striking 
manner in the young growths of Lyginodendron Oldhamium. 
In Plate 25 of my Memoir IV., fig. 16, I represented a young lateral branch, 
bursting outwards thi-ough the cortex of an older stem. The development of these 
branches was demonstrated in several other figures in the same memoir. I havm 
through many past years accumulated numerous additional examples of these young 
branches, which now enable me to speak decisively respecting their structure and 
development. 
Plate 13, fig. 10, exhibits a transverse section of one of these young branches as it 
emerges through the cortex of the parent stem, and before it has become invested by 
an independent cortex of its own. Its centre, a, appears to consist of a solid cluster 
of barred tracheae, of variable diameters, grouped in no special order. Springing 
directly from the periphery of this central mass, a considerable number of tracheseal 
laminae, h, radiate outwards, in regular order, constituting the xylem zone ; these 
laminae are separated by numerous conspicuous medullary rays—both being obviously 
the result of a process of exogenous growdh. Fig. 11 represents a second specimen, 
similar in most respects to fig. 10, but difters, first, in being of larger dimensions (both 
being drawn to the same scale), but, secondly, in a change which is taking place in its 
vascular centre, tlie tracheae composing which axis are now becoming dissociated, 
producing an irregular cavity, a, which is forming in their midst; by this change 
the tracheae are separated into four or five irregular groups, a'a', each of which 
adheres closely to the inner border of the exogenous xylem cylinder, b. Such 
examples show no trace of the proper bark of the branch ; but in another specimen 
in my cabinet (f'abinet number 1885 D), the gi*owth of wdiich has advanced slightly 
beyond that of fig. 11, and in which the cavity, a, has increased yet more in size, 
the characteristic cortex of Lyginodendron is fully developed, showdng that this 
branch had completely emerged from the outer cortex of the j^arent stem. 
In fig. 12 we have a third similar, but yet larger, section, in wdiich the central 
cavity, a, has not only undergone a further increase of area, but is now occupied by a 
* Some of our older botanists scarcely realise tLe progi’ess made during tlie last few years in the 
study of what may be designated “anomalous” features in the tissues of living plants. Such 
“ anomalies” are less rare, even now, than many botanists are aware of. In the primteval plant-world 
they abounded. (See D. H. ScOTT, “ On some Recent Progress in oui' Knowledge of the Anatomy of 
Plants.” ‘Annals of Botany,’ vol. 4, No. 13, p. 147, November, 1889.) 
