382 FILICINEAE LEPTO SPORANGIA TAE 
The growing tips of stem and leaf are often protected by brown 
scales, which are mere trichomes or superficial outgrowths. 
The leaf is usually large, with apical growth and circinate (coiled) 
vernation. The growth often lasts for a long time, or even perma- 
nently (Lygodium). The leaf blade is usually branched pinnately. 
The reproductive organs are borne upon the leaves. The unit is 
the sporangium or spore capsule, a small rounded body, stalked in 
orders 5 and 6 (below) but sessile in the others. The capsule has a 
wall one cell thick, and in this is a group of cells with peculiarly 
thickened cell-walls, termed the annulus , by whose agency (its cells 
being hygroscopic) the opening of the sporangium is effected. Some- 
times, as in many Polypodiaceae, the opening is explosive, and the 
contained spores are violently ejected. The mechanism is in prin- 
ciple similar to that by which anthers dehisce or certain seed-capsules 
open. The annulus may have various forms (see the orders, below) 
but the commonest is that of a row of cells running round the sporan- 
gium for about § of its circumference. 
The sporangia are usually collected into groups termed sort. The 
sorus may be naked, but is more usually covered by an indusium . 
In some cases, e.g. Pteris, this is merely a fold of the leaf itself, but 
more commonly it is a special outgrowth from the leaf, either epider- 
mal or derived from the more deeply placed layers of tissue as well. 
The sori are usually found on the veins of a leaf, often in the angle 
where a vein forks. They do not as a rule occur on all the leaves. 
Very often certain leaves are fertile, the others not. In this case the 
fertile leaves have usually no green tissue at all, their pinnae being 
entirely covered with sori, e.g. Osmunda sp. In other cases, e.g. 
Aneimia sp., one part of a leaf is sterile, the other fertile. Or again 
the sori, and this is most common, may be borne simply on the 
ordinary leaves. They are almost always on the lower surface only; 
they may entirely cover it, but more often are localised. Into the 
vexed question of the evolutionary origin of sporangia we cannot enter 
here. 
The spores are all of one kind and if sown under suitable condi- 
tions give rise to prothalli\ these are flat green expansions living 
for a short or long period independently upon the soil (numbers of 
them may be seen wherever ferns are growing). On the under surface 
are borne the reproductive organs of both sexes, antheridia (male) and 
archegonia (female). The spermatozoids swim to the ova in the 
water which collects under the prothali during rain. The fertilised 
ovum developes directly into a new fern- plant. 
Two interesting modifications of the life cycle as above described 
are known. In Pteris cretica } Nephrodium Filix-mas , Aspidium 
falcatum and Todea africana , there occurs apogamy or the omission 
of the sexual process from the life-history (see diagram in art. Pteri- 
dophyta). The new fern-plant is produced from the prothallus by a 
