370 BOTANY 



Agaves, next to the Cacti, are the most conspicuous plants. Para- 

 sites and saprophytes are of rare occurrence among the Mono- 

 cotyledons, and are confined to the Orchidaceae and the related 

 Burmanniaceae. 



The Gametophyte 



With few exceptions, the gametophyte conforms to the ordinary angiospermous 

 type. The ripe pollen-spore contains either one or two generative nuclei, besides 

 the single vegetative nucleus. In addition to these three nuclei, there have been 

 observed, in Sparganium simplex, and exceptionally in Lilium tigrinum, a small 

 sterile cell, which possibly represents a prothallial cell like that in the micro- 

 spores of Selaginella. A division of the pollen-tube nucleus has also been re- 

 corded for Lilium auratum. 



The embryo-sac may arise directly from the primary hypodermal cell. This 

 is the case in many Liliacese. More commonly the cell divides, by a transverse 

 wall, into an outer tapetal cell (Fig. 319) and an inner one, which may de- 

 velop at once into the embryo-sac, or may divide into a series of cells, one of 

 which destroys the others, and becomes the embryo-sac. 



In Ariscema triphyllum, and this not improbably may be found in some other 

 Aracese, after the tapetal cell is cut off, the archesporial cell divides longitudi- 

 nally into four cells, one of which grows faster than the others. This cell divides 

 once more by a transverse wall, and the lower cell is the embryo-sac. 



The complete gametophyte ordinarily shows the typical structure found in 

 Angiosperms ; but in many Grasses the three original antipodal cells generally 

 increase in number, sometimes thirty or more being found in the embryo-sac at 

 the time it is fertilized. In abnormal cases in Naias and Zannichellia and in 

 some Aracese indications of an increased number of nuclei in the unfertilized 

 embryo-sac have been observed, but these are all exceptional cases. Further 

 research in the lower Monocotyledons will probably bring to light other departures 

 from the typical structure. 



Pollination 



Pollination may be effected by the wind (Palms, Grasses, etc.), by 

 water, or by insects. The adaptations for water pollination are of two 

 kinds. In forms with submersed flowers (Zostera, Naias), the pollen 

 is thin-walled, and in the former extremely elongated, so that the 

 pollen-grains readily attach themselves to the stigma when they 

 come in contact with it. In Vallisneria (Fig. 361) the pistillate 

 flower opens above the surface of the water; and the minute male 

 flowers break away from the submersed inflorescence, and rise to the 

 surface, where they expand and float about until the open anthers 

 come in contact with the stigmas of the female flower, upon which 

 the pollen is deposited. 



Monocotyledons with showy flowers, like the Lilies, Iris, Orchids, 

 etc., are entomophilous (insect-pollinated) ; and some, like species of 

 Iris, and many Orchids, are quite dependent upon insects to insure 

 pollination. 



Sometimes a long interval elapses between pollination and fertili- 



