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likely to be exposed to the air and therefore equipped 

 for the part. It is a significant fact that in those species 

 producing but few stomata, they are located on the tips 

 of the leaves in the very position to be of most service 

 should any part of the plant be exposed. Stomata, then, 

 it would seem, are not of great diagnostic value, but their 

 comparative abundance should be a fairly accurate index 

 to the depth of water usually inhabited by the plant. 



Leaf Characters. — All plants, except segregates by 

 radical botanists, are bound to vary in size with the kind 

 of soil they inhabit, and up to a certain point the number 

 and length of the leaves may be taken to indicate the 

 comparative vigor of the specimens. We may distinguish, 

 however, between the size that is characteristic of a 

 species and the size that is due to the vigor of individuals. 

 In other plants mere size seldom enters into consideration 

 as a specific character, but among plants so closely alike 

 as the Isoetcs it may well be taken into account. As to 

 the color of the leaves, the difference in the shades of 

 green is probably of little significance, but the occur- 

 rence of red in some species may be of more importance. 

 It is well known that the sea-weeds inhabiting the deeper 

 waters are not green but are various shades of red and 

 purple. This color acts like the leaf-green in the economy 

 of other plants. Possibly it may be of similar service to 

 Isoetes. It is noticeable that it occurs most frequently 

 in the species inhabiting deep waters where the ordinary 

 chlorophyll is less able to act. 



Indusium. — The sporangia are borne singly in a hol- 

 low on the inner side of the leaves at base and the tissue 

 of the leaf grows over the sporangium in such a way as 

 to partly or wholly conceal it. This outgrowth is called 

 the velum or indusium. It is much more variable than 

 the indusia of ferns, and seems of little value in exact 

 work. In certain species, however, the sporangia appear 

 to be always covered by the indusium which may thus 

 be of minor importance in distinguishing species. 



Sporangia. — In many species certain cells of the 

 sporangia walls become thickened, giving them a spotted 



