PTERIS AQUILINA. 109 



4. The nodes and internodes of the stem; the nodes are indi- 

 cated by the growth of a leaf at each, alternately on the 

 right and left sides : the intervals between the nodes are the 

 internodes. 



5. Draw. 



II. THE LEAF. Observe: 



1. The leaf-stalk, or petiole, arising from the rhizome and fre- 

 quently miscalled the "stem"; its strength and general 

 appearance. 



2. The system of branching. 



3. The leaf blades, or leaflets. 



4. The arrangement of veins in the blades, venation. 



5. The sporangia, and the folded edge of the leaflet that 

 protects them. When sporangia grow in clusters each 

 cluster is called a sorus (pi. son}. When there is an epi- 

 dermal outgrowth above a sorus it is called an indusium. 

 In Pteris the folded leaf margin is a false indusium. 



6. Draw. 



MINUTE STRUCTURE. 

 I. THE STEM. 



Make a thin transverse section of the stem and study the 

 general regions by means of the low power, and the cell 

 structure by means of the high power. Observe: 



1. Epidermal region, consisting of a single layer of cells. 



2. The sclerenchyma, the heavy -walled strengthening tissue 

 beneath the epidermis. 



3. Within this outer layer of sclerenchyma, the irregularly 

 semicircular fibrovascular region enclosed by a layer of 

 cells, the bundle-sheath. Within the bundle is composed 

 of very heavy- walled cells (xylem), and others (the phloem} 

 with much thinner walls. Study xylem and phloem cells 

 and observe their distribution with reference to one an- 



