Arum maculatum from the Seed . 405 
surrounding cells, and are nearly filled by dense masses of 
needle-shaped crystals. Thick-walled secretory sacs are also 
found in the tissues of the cotyledon and of the older leaves. 
Occasionally they even occur in the epidermis. On treatment 
with salts of iron their contents become black, showing the 
presence of tannin. We have not found tannin sacs accom- 
panying the vascular bundles. 
Before passing on to another part of the subject, we may 
mention a third point in which the anatomical characters of 
these seedlings throw light on their external form. The 
contractile roots of older specimens (Fig. 8) are remarkable 
for the radial elongation of the cells belonging to the inner 
cortex. The appearance of the central cylinder in transverse 
section is in no way distorted by the consequent contraction 
of the whole root, but the cells of the outer cortex are very 
much crushed and strained l . Sections from the ribbed roots 
of third or fourth year plants show these peculiarities most 
plainly. 
Hitherto we have dealt only with those points in the 
anatomy of Arum seedlings which serve to complete our 
knowledge of their external form. For this purpose we have 
referred briefly to their vascular system. But the detailed 
study of this system is interesting in itself, and of importance 
from two points of view, which we will consider separately. 
The course of the bundles in the mature tuber of Arum 
cannot be deciphered. It is always much more difficult to 
follow the vascular system in a shortened stem than in one 
which possesses internodes of moderate length. Two other 
complications render the task impossible in this case. All 
the bundles of the central cylinder form a continuous vascular 
girdle at each node. This peculiarity is found in many 
genera of the Aroideae 2 . But besides this, all the bundles in 
the tuber anastomose freely with each other. This is the 
case even with the leaf-traces which are passing downwards 
through the cortex to enter the central cylinder at one of the 
1 Rimbach, 1. c. 
2 De Bary, Comp. Anat., Eng. ed., pp. 268-269. 
