34 THE PETJIT OF OPTJNTIA TITLGIDA. 



range outside the protected, imgrazed property of the Desert Laboratory 

 might show that seedlings arose from seeds that had passed through the ali- 

 mentary canals of cattle that had eaten the fruits. This search also proved 

 futile. It is of course possible that seedlings may occur in other parts of 

 the area of distribution of this opuntia, or in the Tucson region itself, at 

 other seasons. 



With the hope of discovering something regarding the cause or causes of 

 this failure of the seeds to germinate naturally, the attempt was made to 

 germinate them artificially. In the first place, dozens of plump seeds likely 

 to possess good embryos were repeatedly, in spring, autumn, and winter, 

 sown on soil or on filter-paper, and put either in a warm greenhouse or on a 

 warm bath at 30° to 35°. Not a single germination was obtained from 

 these experiments. 



Other series of sowings were made of plump and apparently fertile seeds 

 from which the seed-coat had been cut or filed away at one part of the 

 margin. Out of several dozens of these cut seeds sown between layers of . 

 damp filter-paper in a gallon battery-jar, only about 8 or 10 germinated. 

 The best proportion of germinations obtained was 5 embryos out of 25 prob- 

 ably fertile seeds. 



The difficulties in determining what proportion of seeds are capable of 

 germination are two. In the first place, it is impossible to tell from its 

 external appearance whether a seed has a normal, well-developed embryo or 

 not. Quite aside from the half -developed and withered seeds, there are 

 many of mature size, and apparently plump and healthy, which upon being 

 cut open reveal no embryo at all, or a discolored and shriveled one with no 

 stored material in it. Secondly, of the seeds that are cut into far enough 

 to determine whether they contain good embryos, or far enough to make 

 certain the escape of the swelling embryo from within, some may be so 

 injured as to prevent germination of any sort. 



The trials made thus far show that a certain relatively small percentage of 

 the seeds of this opuntia are fertile and capable (if aided in getting out of the 

 coat) of producing normal seedlings (fig. Y6). 



