[Vol. 9 



84 ANNALS OF THE MISSOURI BOTANICAL GARDEN 



GENERAL MORPHOLOGY 



The genus limits of Isoetes are very sharp and clear, so that 

 there is no difficulty in recognizing a member within it. How- 

 ever, sections within the genus prove more difficult of discovery 

 and definition. In the past, much emphasis has been laid by 

 systematic workers in this field on the relations to water. En- 

 gelmann used the subdivisions "submersed, amphibious, and 

 terrestrial" as his primary subdivisions. Motelay and Ven- 

 dryes used two main headings, thus: 



I. Aquaticae 



Submersae 



Palustres 



Amphibiae 



II. Terrestres 



It has seemed that the ecological relations might not be ade- 

 quate as bases for primary divisions of the genus. The attempt 

 has therefore been made to utilize more constant features of a 

 morphological character. It seemed that the spore characters, 

 related as they are to fruiting rather than to vegetative stages, 

 might prove more consistent. It has been found that the mega- 

 spores, especially, run fairly constant in size and markings. In 

 conjunction with these spore characters, it is necessary for safety 

 in classification to take into consideration other features, such as 

 the velum, which proves fairly dependable as a character, the 

 lobing of the plant corm, usually a reliable feature, and other less 

 constant characters like the ligule and the presence and number 

 of peripheral strands of supporting tissue in the leaf. These 

 features are considered in greater detail in the subdivision which 

 follows, i. e., morphology. 



In the genus the habit of the body is definitely characteristic 

 and easily recognized in perfect specimens. The stem portion 

 is unusually compact, both vertically and horizontally, and gives 

 rise to a group of rush-like leaves in crowded spiral formation, 

 and to many dichotomously-branched roots. 



Stripped of its leaves and roots, the perennial stem is readily 

 seen to have two or three more or less deep furrows, which result 

 in producing a two- or three-lobed body. The number of lobes 

 so produced is characteristic of the species concerned, save in 

 the rare cases where three lobes may be found in a usually bilobed 

 form or four in a trilobed. The roots appear chiefly in the fur- 



