204 
§ 216. The excentricity of the pith in Rhus Toxicodendron.— 
If we examine a cross-section of the stem of the Poison Ivy, we soon 
find that the excentricity of the pith is due to both the greater num- 
ber of cells on the supported side and their larger size. Tine ducts 
particularly are much larger on the supported side, while there is no 
such marked difference in reference to the prosenchymatic wood- 
cells. In the sections on my slides the average diameter of the ducts 
on the smaller side is .o4 m. m., while on the larger side it is .11 m. m. 
The prosenchymatic wood-cells measure on both sides from .o13 to 
.o14 Mm. m. 
As the lateral rootlets undoubtedly play an important part in the 
life of our plant, I examined a considerable number of sections from 
such places where those roots are inserted in the body of the stem. 
None of them are merely superficial, but they all arise more or less 
deeply in the stem, at the outside of some layer of the fibro-vascular 
bundles, piercing, therefore, both the concentric circles of the cam- 
bium and the phloem or bast layers. Their ends within those woody 
tissues are conical or club-shaped, often bottle-shaped, and they 
are firmly wedged in between the wood-cells and ducts. Invariably 
one of the medullary rays terminates at the extremity of the rootlets. 
They are formed, as all roots, of some outer layers of wide, thin- 
walled, parenchymatic, and of a body of long, prosenchymatic wood- 
cells, with dotted ducts among them; but I could not find any 
pith. ‘The outer layers of their cells are in close connection with 
the adjoining cells of the stem, which they meet at different angles. 
This connection can be noticed very plainly, especially on longitu- 
dinal sections parallel to tangents. 
After this short anatomical examination we are perhaps better 
prepared -to discuss the physiological question. What makes the 
woody tissues nearest the support of the plant grow so much more 
vigorously than those on the opposite side ? 
Before trying to answer this question, allow me to state briefly, 
what our best authorities, such as Dr, Jul. Sachs, think about the 
process of cell-growth in general. No cell can grow, unless it is 
sufficiently supplied with water. But here we have to distinguish 
between cells with perfectly closed, usually thin walls (such as pro- 
bably all young cells have), and cells whose walls are perforated (for 
instance, aan In the first case, the water, by endosmose, enters the 
lumen of the cell and completely fills it. But the endosmotic force 
not only fills the inner space of the cell, but causes a powerful ten- 
sion of its walls, the so-called turgor. This turgescence cannot take ° 
place without removing the molecules, forming the cell-walls, from 
one another, and it is at this stage that particles of new matter, kept 
in solution in the water, are deposited between the molecules, so that 
afterwards, even when the turgos subsides, the cell retains its greater 
size. In cells with perforated walls there can, of course, be no tur- 
- gescence, and here it is the imbibition of water by the thick cell- 
walls themselves that makes them turgid, whereupon, as in other cell- 
_ walls, interposition of new material causes the growth of the cell. 
If we accept this theory, and I think we must, for want of one 
more plausible, we have to believe, that without turgescence or im-_ 
