2 6 
PLANTS. 
this curious mechanism is very evident in the com- 
mon brakes, and in the hart’s-tongue ; but the 
sudden jerk of the springing cord frequently car- 
ries the seeds out of the field of the microscope, 
and sometimes defeats the purpose of the observer. 
The common horsetail, which pushes up its solitary 
club-shaped head in the spring, affords another in- 
stance of this singular mechanism. The seeds of 
this plant are minute oval bodies, with two, three, 
or four slender threads attached to each. If the 
ripe fern be shaken over a piece of white paper, a 
greenish powder falls out, which at first appears 
full of animation, but soon becomes quiet. This 
powder is the seed, and the motion is occasioned 
by the contraction of the elastic threads ; for a good 
microscope readily discovers all this, and exposes to 
our view the little oval bodies with their slender 
threads. Moisture will immediately put the whole 
collection into motion ; the slightest breath will 
make the threads coil about the seeds, but in a 
moment, becoming dry, they expand again : after 
several expansions and contractions, they detach 
themselves, still contracting when moistened, and 
gradually bending from a straight line into a circle. 
They are so very susceptible of moisture, that, Dr. 
Withering informs us, if a drop of water be pushed 
toward them, they will contract before it touches 
their bodies. This property is so very singular, that 
a person unacquainted with the cause might readily 
be excused for supposing each seed a living insect. 
Some seeds assist their projection to a distance in 
