62 
F STOWARD, D.SC. : 
Generally, if the section is carefully examined, we find 
here and there among the parenchymatous cells other 
cellular units, oval or circular in cross section, possessing 
thick walls with transverse markings, and presenting in their 
superficial view a pitted appearance. The walls of these 
cells, when stained according to Tison’s method, assume a 
magnificent blue colour. Their resemblance to the stone 
cells met with in the pulp tissue of the pear is striking. 
These cells bear the distinctive name of “ sclerotic cells ” 
or “ stone cells ” (Fig. 3c). 
Their oiigin, nature, function and possible significance 
is, for the present, a matter of conjecture. At one time 
it was supposed that the presence of such cells was an in- 
dication of advanced ripeness. The fact that map} 7 pie 
met with in quite young tubers, however, renders this view- 
untenable. Appel regards these cells rather as tepiesenta- 
tive of a varietal characteristic and very reserved Iv suggests 
that their pieserce may be accepted as a possible indication 
that the varieties of tubers in which these units are present 
have arisen directly from sclerotic types, or possibly by 
hybridisation from sclerotic types. This view, whatever 
may be its ultimate value, evidently suggests an hereditary 
origin for these sclerotic cells. 
Lastly, we come to the vascular elements (Fig. 2v) r 
which, in most sections, appear as shreds and patches due 
to the fact that these structures are irregularly disposed in 
relation to the plane of the section. These vascular strands 
are tabular structures, forming a more or less intricate con- 
ducting system, and representing an extension of similar 
structures in the rhizome. They may be traced to the 
vascular system of the above ground parts of the plant. 
If carefully investigated by means of serial sections, it will 
he found that branches of this root system of vessels can be 
traced to each eye ” of the tuber. I he vascular stiands 
are the highways along which the translocation of the 
metabolic products formed in the leaves of the plant takes 
place, the most important of which are the carbohydrates, 
which are ultimately stored in the tuber as starch. 
During the grow th and development of the young crop 
tuber the processes which predominate are essentially those 
of mobilisation. During the period of after ripening, fol- 
lowed by the onset and continued progress of sprouting, 
the chief phenomena are those of immobilisation. The 
starch and also probably the proetin storage materials undei 
the influence of certain specific enzymes become, simplified 
and solubilised and thus rendered available for the nutrition 
of the growing sprouts. 
