m 
UPWARD GROWTH OF THE HEAD, AND [Part II. 
induce the stems to grow downward to the light, 
and the roots upward in the earth, some of 
the gemmules or stems grew upward, and came 
out of the hole at the then upper part of the 
inverted flower-pot. These plants passed through 
eight inches and a half of earth. On dividing 
them lengthwise I found that, in those parts of 
the stem which grew in the air, the piths and 
herbaceous envelopes were green, and in those 
parts which grew in the earth they were white. 
The piths ran the entire length of the roots as 
well as the stems; and where the necks of the 
plants divided the stems from the roots, the two 
piths were continuous, and of precisely equal 
size. It is the universal error in physiology, to 
believe that roots have no piths. Let any one 
divide a seedling horse-chesnut, and he will con¬ 
vince himself. 
Each layer of bark is supposed to have its 
proper pith or cellular ring outside it. The 
green cellular or “ herbaceous envelope ” under 
the outer cuticle is supposed to be the pith 
of the outer layer of bark, and to be to the 
layers of bark what the central pith is to 
the layers of wood ; and throughout its whole 
extent there is probably a direct vascular com¬ 
munication between this green external pith of 
