of Lysigenous Cavity -formation. 4 1 1 
turgor is withdrawn, it follows that in those stems in which 
the extension of the pith, or, better, the positive tension of the 
pith, ceases with the end of primary growth, there must be 
a contraction of the pith as the latter loses more and more of 
its turgor. When the negative tension thus called forth is 
considerable, schizogenous clefts may precede the collapse of 
cells. 
In those plants like Sambucus and Helianthus tuberosus , in 
which the pith dies during secondary growth without col- 
lapsing, it is probable, as found true by Kraus for the two 
plants mentioned, that the pith is in the condition of positive 
tension for some time after the beginning of secondary growth. 
When the cells lose their turgor, the force of contraction is 
not sufficient to separate them. 
4. Effect of Prevention of Tension. 
Many years ago Sachs 1 found that leaves of Allium Cepa 
grown in the dark were not hollow ; he did not, however, 
describe the difference in the histology of normal and etiolated 
leaves. It is easy to understand the immediate cause of the 
normal formation of the cavity when observing that the 
etiolated leaves have in cross-section a semilunar shape, with 
peripheral cells slightly oblong but not of the well-known 
H-palisade form, while the inflation of the leaf goes hand in 
hand with the rapid growth and consequent tangential enlarge- 
ment of these peripheral cells. In this case then, when we 
prevent this inflation of the leaf by growing it in the dark, we 
prolong the life of the central parenchymatous cells. Many 
individuals have been thus grown in the dark, and leaves 
a foot long produced altogether of living cells, whereas 
normally the leaf becomes hollow at a distance of five to eight 
centimetres from the point of its emergence from the bulb. 
If zones of these leaves are etiolated by opaque wrappings 
while the rest of the leaf is exposed to the light, the leaf will 
1 Sachs, Ueber den Einfluss des Tageslichtes auf Neubildung und Entfaltung 
verschiedener Pflanzenorgane. Bot. Zeitung, 1863, Beilage. 
F f 2 
