ANATOMY OF TISSUES. 129 
parallel to that of the stem and somewhat sharpened ends, and 
he in tangential rows of varying number. The middle row or 
rows are supposed to generate the new cells, but this is a very 
difficult matter to decide with certainty. The ducts are nearly 
all porous and the pores are bordered. Spiral ducts are very 
rare, but are found in certain Cacti forms. The primary bun- 
dles often develop these in such numbers as to form a sort of 
ring around the pith known as the pith crown. Tracheids are 
numerous, generally with small slit-formed pores like those of 
the libriform cells. The wood of Conifers is peculiar in that it 
produces no ducts in the secondary growth, their place being 
supplied by tracheids with considerably thickened walls and 
large bordered pores. The axis of the wood-parenchyma cell 
which is parallel with the long axis of the stem, is somewhat 
longer than the other two though not enough so to render it 
prosenchymatic. These cells generally occur in groups, and 
are situated not far from the ducts. Sometimes they surround 
them in a circle. Sometimes they form a nearly continuous 
circle around the stem. ‘Together with the medullary ray cells 
they form the living elements of the wood. ‘They originate from 
a prosenchymatic mother cell, which divides by anticlinal walls. 
The libriform cells have already been characterized in the de- 
scription of the compound bundle. 
The elements of the secondary phloem correspond to those 
of the primary, that is, they consist of bast fibers, sieve tubes 
and parenchyma. Besides these there are often groups of stone 
cells, and glands containing calcium oxalate crystals which often 
accompany the bast fibers. These last generally occur in groups. 
In some plants they are very long, in others shorter. The older 
layers of phloem generally have more bast than the younger, its 
formation often decreasing so that in later years only small 
bundles of it occur. Very few rinds are free from bast, while 
in many stems of trees the bast fibers build a closed cylindrical 
mantle. 
