—43— 



called terrestrial species? If the plants are to be divided 

 as to habitat it would seem that there should be a new 

 distribution. In all probability two divisions, Aquatic 

 and Amphibious will be quite sufficient. In our own plants 

 at least the so-called terrestrial species would better be 

 called winter and spring species. In the case of /. Mel- 

 anopoda, one of the " terrestrial " species, Engelmann 

 has shown that if kept covered with water it lasts through 

 the summer like other species, but if not its leaves die 

 early in the season. Much more extended observations 

 of the various species both in winter and summer are 

 greatly needed. 



The Trunk. — Baker in his " Fern Allies " uses the 

 number of lobes in the trunk as a convenient means of 

 separating the species in the various sections, but this 

 is scarcely a satisfactory character. Isoetes is like the 

 Dicotyledons in producing a central cylinder surrounded 

 by a ring of growing cells the innermost of which 

 annually add to it on one side while the others produce 

 bark. At certain points these cells happen to be more 

 active and thus a bulge or lobe of the trunk is produced. 

 This lobing is fairly constant for each species, some hav- 

 ing two lobes and others three. Some species, however, 

 are indifferently two-lobed or three-lobed and plants with 

 more than three lobes are known. Since less than half a 

 dozen of our species are regularly three-lobed, the lobing 

 of the trunk is not of much assistance in their identifi- 

 cation. 



Stomata. — Stomata are the breathing pores of plants ; 

 that is. they are openings through which certain gases 

 pass to and from the interior of the leaves. In water 

 plants the submerged leaves bear no stomata and the float- 

 ing leaves show very plainly the relations of these open- 

 ings by bearing them only on the upper surface where 

 they are exposed to the air. It is reasonable to assume 

 then, that one of two things is true of Isoetes: either the 

 normally submerged species bearing stomata are the des- 

 cendants of older species that lived exposed to the air for 

 a part of the time, at least ; or they are species that are 



