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THE GENUS ISOETES IN NORTH AMERICA. 457 
is apparently always dicecious, no moncecious plants having been discovered among about one hun- 
dred examined. In J. melanopoda I have sometimes seen leaves with microsporangia irregularly 
interspersed among those that bear macrosporangia. 
‘The macrospores are little spheroid bodies between one-fourth and three-fourths of a millimetre 
in diameter. Their surface is divided by a circular rim into a lower hemispherical and an upper 
three-sided pyramidal part, the three faces of which consist of spherical triangles and are separated 
from one another by three elevated ridges. The crusty surface of these spores, chalky-white or 
whitish in most species and dusky (when wet black) in Z. melanospora, is rarely smooth, but gen- 
erally sculptured and differently marked. The three upper triangles are sometimes marked differ- 
ently from the lower hemisphere (especially in J. Tuckermani) or are smoother than that (often in 
I. melanopoda). To examine the spores well it is necessary to soak the leaf-base, carefully remove 
some of the wet spores and let them dry on the slide, for they must be examined dry, and best 
under a power of 50 or 60 diameters; but, to study the sculpture well, a power of 100 to 150 di- 
‘ameters is necessary, With the aid of this we find the macrospores —1. Minutely tuberculated or 
warty ; the warts small and mostly somewhat depressed, distinct or sometimes somewhat confluent, 
in I. pygmea, Bolanderi, saccharata, melanospora, Butleri, and Nuttallii. 
2. With larger, broader tubercles, generally more distant and distinct, but also here and there 
confluent, worm-like ; thus in I. flaccida, melanopoda, and Cubana. 
3. With tubercles elongated into spines; these are simple and very fragile, or here and there 
confluent and forming sometimes short crests: J. echinospora and its forms. 
4, With crests and ridges, distinct or anastomosing: J. lacustris, Tuckermani, and [370 (13)] 
riparia. 
5. The confluent crests form a regular network: J. Lngelmanni. 
The microspores are minute bodies of an ash-gray or a dusky color (dark gray in LZ. pygmea, 
Bolanderi, and melanopoda, deep brown in I. melanospora, Butleri, and Nuttallii) and of a somewhat 
triangular-oblong shape, nearly straight on one and curved on the other two edges, more than half 
as wide as they are long, between 0.020 and 0.040 millimetres in the longest diameter. Their sur- 
face is smooth or minutely papillose or spinulose, the edges smooth or somewhat cristate. Their 
size furnishes good characters, but the condition of the surface much less so. They ought to be 
examined under water and with a power of about 400 diameters. 
§ 3. BrorogicaL CHARACTERS. 
After the maturity of the spores the leaves wither or rot away, the sporangia decay and set the 
spores free, which scatter near the base of the plant, often being retained between the matted roots.* 
The cellular mass of the macrospores develops into a prothallus, which bursts the spore-case through 
the opening of three valves which correspond to the three upper faces of the spore, and forms an 
archegonium, which is fertilized on coming in contact with the zodspores emitted from the micro- 
spores, and thus gives rise to the young plantlet whenever moisture and temperature favor this 
process. 
The germination of the late-maturing water-species probably takes place in the succeeding 
spring, at least in the more northern localities; in our land and marsh species it may be observed 
soon after their maturity in summer or in early autumn. 
I have studied the whole process in Z. Engelmanni, which I kept in cultivation for ogee 
years. At the end of July the spores were perfectly mature and the leaves were coming off. 
the 28th of that month I spread out both kinds of spores on a muddy surface, and kept them slightly 
4 It is therefore proper to examine among the roots for spores whenever none can be found on the plant ; one or the 
other may be discovered there, and help out the diagnosis, big otherwise may rest in obseuri 
