6o 
F. STOWARD, D.SC. : 
tubers these units are rarely present. Each “ eye,” as 
already stated, is a collection of small buds ; in fact, it is 
virtually a lateral branch with undeveloped intenodes, such 
as we might imagine if on an ordinary stem all the leaves 
with their auxiliary buds were subjected to an upward 
thrust, with the result that all the structures were raised 
to its summit and there closely pressed against each other. 
Thus the whole tuber does not represent, as is sometimes 
popularly supposed, a simple shoot, but a very much 
branched shoot system. 
The Histology of the Potato Tuber. 
So far we have considered the gross or external anatomy 
of the tuber, and have become more or less conversant with 
those characteristics which may be recognised and identified 
by the unaided eye. 
It is essential, in order to become acquainted with 
the minute anatomy or histology of the tuber to examine 
thin slices or sections of it under the lower magnifying 
powers of the microscope. These thin slices may be readily 
enough prepared from fresh material either as hand-cut 
sections or the material after fixing and suitable subsequent 
treatment may be embedded in paraffin and sectionalised 
with a microtome. Assuming that we have such a thin 
section properly mounted, and on the stage of the microscope 
in readiness for examination, then starting from without 
and working inwards, the following structures will be met 
with in the order given :• — • 
(1) The skin (Fig. 3a), consisting of several layers 
of rectangular cells, arranged with their longer 
axis parallel to the surface. 
(2) The phellogen or cork cambium (Fig. 36). 
(3) Transitional cells (Fig. 30). 
(4) The parenchymatous cells (Fig. 3 d), large iso- 
diametric cells, the protoplasm or living sub- 
stance of which is more or less crowded with 
variously sized starch grains and other less 
conspicuous inclusions. 
(5) Strands of vascular tissue and sclerotic cells 
in certain cases, the latter figured in Fig. 3c. 
We may consider each of these various tissues and 
structural units in more detail. Commencing with the skin 
we may note that the cells which constitute its outermost 
layers are apparently devoid of protoplasmic contents and 
inclusions ; and it is doubtful whether these extreme outer 
layers are composed of actively living cells. The investi- 
gation of this point is a matter of difficulty, seeing that 
neither plasmolytic tosts nor intra vitarn staining afford 
evidence which can be regarded as conclusive. The inner- 
