124 MISSOURI BOTANICAL GARDEN. 



nection with the desert species, which possess a firm, 

 parchment-like core immediately about the seeds. 



The pollination relations of nearly all of the group are 

 among the most peculiar and exclusively restricted thus far 

 discovered. Hesperaloe secretes much nectar and appears 

 adapted to birds, as are the Cape aloes, to which it bears 

 no inconsiderable resemblance in its flowers. The other 

 genera are sparingly if at all nectariferous, though all have 

 septal glands, which are rather small in Clistoyucca, but 

 verv large in the others. Yucca aloifolia, again in an 

 exceptional way, appears to be freely self -fertile, but self- 

 seeding is very unusual with all of the other species of 

 this genus, as it appears to be with Hesperoyucca, Clisto- 

 yucca and Samuela. These, so far as known, depend for 

 their pollination upon small moths belonging to the tineid 

 genus Pronuba, of which one species (P. syntlietica) is 

 known only in connection with the single species of Clisto- 

 yucca, one (P. maculata, and its variety aterrima), with 

 the single species of Hesperoyucca, and the only other 

 known species (P. yuccasella) accompanies the various 

 species of Yucca across the continent and has a known 

 north and south range from the great bend of the Mis- 

 souri river to central Mexico. These moths are not known 

 to feed, in the larval stage, on anything but the developing 

 seeds of the plants named ; so that the mutual dependence 

 of moth upon plant and of plant upon moth appears to be 

 absolute, no doubt, taken in connection with the other 

 ecological peculiarities of the yuccoids, a fact of the 

 greatest suggestiveness, but the bearing and meaning of 

 which has as yet escaped both botanists and entomologists. 

 That the flowers were formerly pollinated otherwise appears 

 to be indicated by the presence of nectar-glands, which 

 now appear to be useless. 



The Jong perianth tube of Samuela, a type of struc- 

 ture usually connected with pollination by some insect of 

 corresponding tongue-length, for which the nectar is thus 



