204 
PROFESSOR W. C. WILLIAMSON ON THE ORGANISATION 
diameter of the 'medulla either remains constant or even diminishes, so that in many 
old stems but feeble traces of it can be discovered.''' 
This process is a very different one from what we see in figs. 7—12 of the present 
Memoir. In fig. 7 the growth of the twig in length and diameter has not only made 
considerable progress, but its tissues have developed into a definite central vascular 
* In illustration of this point I select a few examples from a number of measurements which I have 
made from transverse sections of growing stems, the measurements being recorded in decimal parts of 
an inch. 
The plant. 
Tran.sver.se diameter at 
apex of medulla. 
Mean 
diameter 
of the 
Piameter at successive points lower down the stem 
measured from apex of medulla. 
Of branch. 
Of medulla. 
medullary 
cells. 
Distance. 
Stem. 
Medulla. 
Medullary cell.«. 
JEschylus Hippocastaneum 
.085 
.04 
.0007 
6 inches 
.25 
.175 
.0014 
18 „ 
.45 
.175 
.002 
Tydcea . 
.1 
.04 
.0009 
I inch 
.11 
.05 
.0019 
1_ 
125 
.175 
.0019 
21 inches 
.25. 
,2 
.005 
28 
i-U 5, 
.22 
.25 
.006 
Geranium . 
.06 
.05 
.0014 
1 inch 
.225 
.105 
.0035 
4 inches 
.275 
.225 
.0057- 
Old stem 
.4 
2 
.0035 
Elder . 
.050 
.03 
.0014 
4 inches 
.17- 
.12 
.0043 
14 „ 
.25 
.15 
.0043 
51 . 
2 feet 
.6 
.35 
.0057 
11 . 
3 5, 
.55 
.25 
.0057 
These measurements show us that, approxiraatelj, whilst 
In ^sclujlus the medulla enlarges transvei’sely from .04 to .75, 
,, the medullary cells eulai’ge from .0007 to .002. 
In Tijdcea the medulla enlai’ges transversely from .04 to .25, 
,, the cells enlarge from .0009 to .006. 
In Geranium the medulla enlarges transversely from .05 to .2, 
,, the cells enlarge from .0014 to .0057. 
In Elder the medulla enlarges transversely from .03 to .25, 
„ the cells enlarge from .001 to .0057. 
We are thus led to the approximate conclusion that— 
In JEschylus three-fourths of the transverse enlargement of the medulla is due to the expansion 
of the primitive medullary cells. 
In Tydcea all the transverse enlargement of the medulla is due to expansion of those primitive 
cells. 
In Geranium all the transverse enlargement is due to the expansion of the primitive cells. 
In Elder three-fourths of the transverse enlargement is due to the expansion of the primitive 
cells, and not to an increase in their number. 
