ANATOMY OF TISSUES. 63 
contents enclosed by the layer are also protoplasmic in nature, 
but are more dense than the former. Strands of the denser 
material pass through the fine openings of the sieve-plates and 
are therefore continnous from member to member, or as is 
usually said, from cell to cell. This direct communication 
between the cells destroys their individuality ; hence the term 
sieve-tube for the continued row, and member for each original 
cell. 
The sieve-tube of the gymnosperms and vascular crypto- 
gams differs but sightly from the type described above. It is apt 
to be prismatic rather than cylindrical. The sieve-plates occur 
not only at the ends of the tubes but also at the sides wherever 
a lateral union takes place, and they are less frequently changed 
to callous plates. The accompanying cells of cribose tissue are 
long, thin-walled, and sharp-pointed, varying but little from the 
shape of the cambium cells from which they arise. They follow 
the course of the sieve-tubes, are without pores, and always 
retain the character of cells, that is, are entirely enclosed by 
walls. For these reasons, they are supposed to be conducting 
cells for those nitrogenous substances which are easily dissolved. 
The parenchymatic element of phloem is quite similar to all 
ordinary thin-walled parenchymatic tissue. 
Bast cells or fibers are long and thick-walled, with pointed 
ends and simple pores. The name bast was formerly applied 
to the entire collection of tissues where such cells were found ; 
it is now restricted to all cells of this class except those found 
in the xylem part of the bundle. They are sharp-pointed from 
the first, the transverse walls of the cambium from which they 
are cut off being more or less oblique to the long axis, and their 
subsequent growth is such as to increase this tendency. They 
grow in many places to several times their original length, while 
the surrounding tissue does not keep pace with them ; it is 
therefore believed that the sharpened end of the bast cell is able 
to force its way between the lamellae of the walls which would 
