SPOROPHYTE 



429 



{dichotomous branching); and the leaves develop in the spring 

 by unroUing from the base, much hke unrolling a bolt of cloth, 

 untU 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. 



Fig. 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). 



