330 
PLANT STUDIES 
An ordinary fern sporangium consists of a slender stalk 
and a bulbous top which is the spore case (Fig. 296, 6). 
This case has a delicate wall formed of 
a single layer of cells, and extending 
around it from the stalk and nearly to 
the stalk again, like a meridian line about 
a globe, is a row of peculiar cells with 
thick walls, forming a heavy ring, called 
the annulus. The annulus is like a bent 
spring, and when the delicate wall be- 
comes yielding the spring straightens 
violently, the wall is torn, and the spores 
are discharged with considerable force 
(Fig. 301). This discharge of fern spores 
may be seen by placing some sporangia 
upon a moist slide, and under a low 
power watching them as they dry and 
burst. 
215. Heterospory. This phenomenon 
appears first among Pteridophytes, but it 
is not characteristic of them, being en- 
tirely absent from the true Ferns, which 
far outnumber all other Pteridophytes. 
Its chief interest lies in the fact that it 
is universal among the Spermatophytes, 
and that it represents the change which 
leads to the appearance of that high 
group. It is impossible to understand 
the greatest group of plants, therefore, 
without knowing something about heter- 
ospory. As it begins in simple fashion 
among Pteridophytes, and is probably 
the greatest contribution they have made 
to the evolution of the plant kingdom, 
unless it be the leafy sporophyte, it is best explained 
here. 
FIG. ,300. A moonwort 
(Botrychiwri), show- 
ing the leaf differenti- 
ated into foliage and 
sporophyll branches. 
After STRASBUK- 
GEB. 
