150 "TYPE STUDIES 



life ? Describe the life history, distinguishing between the 

 sexual phases, gametophytes, and the asexual phase, sporo- 

 phyte. Draw and arrange a series of diagrams illustrating 

 the chief stages throughout the life history, using two col- 

 ored pencils to designate the gametophytic and sporophytic 

 generations respectively (App. 18). Construct a life-history 

 formula that will express this succession (App. 18). 



136. Isoetes, quillwort. Study when possible the life habits, noting whether 

 the form is aquatic or terrestrial. 



A. General morphology. Note : 



1. The rush-like leaves arranged around a very short, conical stem, and 

 the cluster of forking roots below. Illustrate in a habit sketch. 

 These leaves, late in the season, produce sporangia at their bases, 

 thus becoming sporophylls. At such times the stem may be com- 

 pared to a cone of sporophylls. 



2. Strip the sporophylls from the stem. Those on the outside, and 

 consequently lower on the stem, are likely to be megasporophylls, as 

 shown by the basal sporangium containing megaspores. Those in 

 the interior, and consequently higher up on the stem, are likely to 

 be microsporophylls, as shown by the basal sporangium containing 

 microspores. 



B. The sporophylls. Examine the base of a megasporophyll, viewing it 

 from the inner side. Show in a figure : 



1. The large megasporangium containing megaspores, held in a hol- 

 low at the base of the leaf and partially covered by a membrane, 

 the velum. 



2. The ligule, a triangular scale situated on the sporophyll above 

 the sporangium. Diagram the position of the ligule and sporan- 

 gium as they appear in a lengthwise section of the base of the 

 sporophyll. 



3. Draw the megaspore under h.p., showing the markings on the thick 

 wall and the angles on one side. What do the angles signify ? 



4. Compare the base of a microsporophyll with that of a megaspo- 

 rophyll. 



•5. Draw a group of microspores under h.p. to show their size in com- 

 parison with that of the megaspore. 



C. The structure of the leaf. Section the leaf across and lengthwise. Note 

 the large air spaces separated by partitions. Show their arrangement 

 in an outline drawing and the smMJibro-va^cular 6und/e traversing the 

 interior of the leaf. Are atomata present ? 



