ISOETES 137 



(n) Make drawings as follows: 



(1) A leaf -base, 20 mm. at greatest breadth, 

 showing all points observed under 4, 



(*')-. 



(2) A diagram, 40 mm. in greatest width, of an 

 imaginary cross-section, of the sporophyll, 

 taken through the middle of the fovea. 



(3) A diagram, 15 mm. in greatest width, of 

 an imaginary median longitudinal section 

 through the base of a sporophyll. 



D. Asexual Reproduction: 



1. Note that some of the sporophylls bear large spores 

 (megaspores), and some small spores (microspores) . 

 Study any constant differences (a) in structure, 

 (b) in position on the stem, between the mega- 

 sporophylls and the microsporophylls. Define each 

 of these terms. 



2. How many megaspores does a megasporangium 

 . contain? 



3. Measure the diameter of a megaspore in millimeters. 

 Study and describe its shape under the low power 

 (mounted in water), noting any surface marks. 

 Explain the presence of angles on the spore. 



4. Make a drawing of a megaspore, 20 mm. in diameter. 

 Indicate the amount of enlargement. 



5. Study microspores under high power, describing 

 their shape and surface marks. Draw two or three 

 to the same scale as the drawing of the macro- 

 spore. The number of microspores in I. echinospora 

 is said to be from 150,000 to 300,000; of megaspores 

 150 to 300. 



6. Why is Isoetes a heterosporus pteridophyte ? 

 Compare it with Polypodium vulgare in this 

 respect. 



