402 
PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
portions, we obtain a length of nearly half an inch. Arising, as described, in the 
central axis, it proceeds outwards, penetrating in succession the parenchymatous and 
prosenchymatous layers of the bark, but carrying the innermost pseudocambial layer 
(g) along with it in the form of an investing sheath (</'). At its outer extremity it ter- 
minates in a disorganized carbonaceous mass, which I am unable to interpret. The 
tissues of which this organ is composed remind us of the elongated scalariform cells 
or incipient vessels which form in the developing roots of Ferns and Lycopods. In this 
respect they also correspond with the similar appendage of Dictyoxylon Oldhomium , 
shown in Plate XXIII. fig 7, n'. The above is the only clearly defined example of this 
growth which I have seen in I). Grievii, but unmistakable indications of a similar 
structure appear in Plate XXVIII. fig. 30, n. 
Associated with the ordinary stems of I). Grievii , I find abundance of small twigs of 
the same plant, a transverse section of one of which is shown in Plate XXVIII. fig. 46, 
and a longitudinal section of the same specimen is given in Plate XXXI. fig. 47. The 
former of these is precisely what the bundle 5 of Plate XXX. figs. 41 & 42 would be, 
supposing it could be prolonged upwards, with its investments of parenchyma and pro- 
senchyma, until it projected clear of the parent axis. In the transverse section of each 
•of these twigs we have a more or less triangular figure with prolonged lateral angles. 
The greater portion of this area consists of a mass of parenchyma (Ji), surrounding a vascu 
lar bundle (m) : the latter consists of an intermingled cluster of barred and reticulated 
vessels — often separable into two groups, and imbedded in medullary parenchyma. 
This bundle is not placed in the centre of the twig, but excentrically, or nearer the' 
slightly heart-shaped base than the apex of the triangular outline seen in the transverse 
section, the peripheral margin opposite to it being either rounded or projecting in an 
obtuse angle. The entire structure is invested by a thin layer of outermost bark ( Jc ), con- 
sisting of alternating bands of prosenchyma and parenchyma, like those of the parent 
stem. The thin lateral expansions of the twig are often doubled back upon the thicker 
central portion, as in Plate XXVIII. fig. 46. The position is an accidental result of 
pressure, but it indicates that these angles of the twigs projected like those of a petiole 
with decurrent leaflets. 
On turning to the vertical section (Plate XXXI. fig. 47) we find a single central 
bundle (m) invested with a bark which presents, in an uncomplicated form, the essential 
features of that of the parent stem. We have here the three typical layers of the 
bark, only the inner bark is very feebly represented. It appears as a thin layer of 
delicate, vertically elongated cells, immediately surrounding the bundle ; it is closely 
attached to the central vascular bundle opposite to each of the dark transverse bands 
of the parenchymatous layer, but bulges outwards in the intermediate spaces. The 
moulding effect of contact with this innermost layer is seen in the undulating outline 
of the left side of fig. 45, which only represents the parenchymatous and prosenchymatous 
layers of one of these young twigs. The parenchymatous layer corresponds with that of 
the larger stems, only each of the dark horizontal bands of cells (h!) now extends con- 
