PTERIS AQUILINA. 109 



4. The nodes and internod.es 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. sori). 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- 



