SPOROPHYTE 429 
(dichotomous branching); and the leaves develop in the spring 
by unrolling from the base, much like unrolling a bolt of cloth, 
until their final length is reached (circinate vernation). They have 
epidermis, stomata, and chlorenchyma or food-making tissue, 
and through their veins run well developed vascular bundles. 
Fic. 380. — A cross section of a Fern stem, showing the epidermis (e), the 
cortex (c), the vascular cylinder (v), and the pith (p). 
The sporangia occur in the rusty looking spots, called sori 
(singular sorus), which are formed at certain times on the under 
surface of the leaves (B, Fig. 381). Each sorus has a membrane- 
like covering called indusium, under which the sporangia are 
protected. By making a thin cross section of a leaf, so that the 
section passes through a sorus, the sporangia then appear under 
the low power of the microscope as shown at C' in Figure 381. A 
number of sporangia occur in a sorus, but the number varies in 
different Ferns. The sporangia are usually stalked and flattened, 
and around the margin there is a row of heavy walled cells form- 
ing the annulus, which assists in opening the sporangia and 
scattering the spores (D, Fig. 381). 
