SPOROPHYTE 



429 



{dichotomous branching); aiul 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. 



Fig. 380. — A cross section of a Fern stem, showing the epidermis (e), the 

 cortex (c), the vascular cyHnder {v), and the pith (/j). 



The s-porangia occur in the rusty looking spots, called sori 

 (singular sorus), which are formed at certain times on the uncUn- 

 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 881 . 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 oj^ening the sporangia and 

 scattering the spores (D, Fig. 381). 



