STRUCTURE OF THE STEM 95 
ture. 
stems, such, 
Fie. 67. — Collenchymatous 
andOtherTissuefromStem {for instance, 
1 iens). 
of Balsam (Jmpatiens) as the gTrow- 
e,epidermis;c,collenchyma; . 
iz, intercellular spaces be- in S p oint 
— large parenchyma- between the 
two rudi- 
mentary leaves of a bean-plumule, 
the cells are all of thin-walled 
formative tissue and look a good 
deal alike. This condition of 
things is quickly succeeded by 
one in which there is a cylinder 
(appearing in cross-sections of the 
stem as a ring) of actively growing 
tissue x (Fig. 68, A), lying between 
the cortex rand the pith m. Soon 
the cylinder z develops into a 
series of separate fibro-vascular 
bundles arranged as shown in 
Fig. 68, B, and these again in a 
short time unite, as shown at C. 
A comparison of this last portion 
of the figure with that of the 
chymatous tissue. In some (particu- 
larly in fleshy) stems the stiffness is, 
however, largely due to collenchyma, a 
kind of parenchyma in which the cells 
are thickened or reinforced at their 
angles, as shown in Fig. 67. 
107. Early History of Stem-Struc- 
In the very young parts of 
Fic. 68. — Transverse Section 
through the Hypocotyl of the 
Castor-Oil Plant at Various 
Stages. 
A, after the root has just ap- 
peared outside the testa of the 
seed; B, after the hypocotyl is 
nearly an inch long; C, at the 
end of germination; r, cortex 
(undeveloped bark); m, pith; 
st, medullary rays; jv, fibro- 
vascular bundles; cb, layer of 
tissue which is to develop into 
cambium. (Considerably mag- 
nified.) 
