

SHAVINGS EXAMINED MICROSCOPICALLY. 565 
grows, the number of cells is | ann iplied, and as the growth 
is faster in one direction than in others the resniting cells 
are elongated ; in fact we find in woody tissues that the so- 
called wood cells are more or less fibrous, so that such 
tissue is known as woody fibre. These wood-cells are 
pointed at both ends, in fact are fusiform. Some of the 
cells, however, become united by the absorption of their 
contiguous walls, so that continuous tubes are formed. 
These tubes are for the purpose of transporting the life-blood 
of the plant (the sap), which like the blood of the animal, 
is the source of the new tissues which are built up from its 
matter. As these tubes are of such importance in the 
economy of the individual, it becomes necessary that they 
should be protected from injury, and such injury is most 
likely to be a crushing from without and a consequent stop- 
page of the flow of the sap. If we were to stop the flow 
. of the blood in the arm, for instance, by tying a ligature 
above the elbow, we should find that disorganization of the 
tissues in the fore-arm and hand would result; they would 
mortify and death of the parts would follow. The same 
thing we can readily understand would take place in the 
plant, should the sap-flow be arrested in any way. To pre- 
vent such a disaster these long tubes are strengthened in a 
very remarkable manner, namely, by having a deposit of 
tough lignine formed within their walls, and arranged in the 
form of a spiral. The same mode of structure is to be seen 
in the tubes called traehes, which convey the air to and 
from the lungs of animals. Insects exhibit this structure in 
a very striking manner; the trachez of a caterpillar of some 
ind, most commonly the silk-worm, is & favorite micro- 
scepic object. The spiral arrangement at the same time 
permits of a certain amount of elasticity in such vessels, as 
is to be seen in a very common illustration of such structure. 
I allude to the flexible tubing used to convey burning gas 
from a chandelier to a burner upon the table. Such spirál 
ducts, as they have been named, are to be seen in most cross 
