THE FRUIT OF OPUNTIA FULGIDA. 33 



than nine-tentlis of the thickness of the coat surrounding the seed and con- 

 sists of three distinct layers. The inner layer is from 150 to 300 microns 

 thick. It consists of intertwisted threads or fibers which run meridionally 

 about the seed. The component cells of these threads are 8 to 10 microns in 

 diameter each by 150 or 200 microns long. The middle layer of the funi- 

 cular pocket is made up of somewhat similar interwoven threads running 

 equatorially, i. e., around the luargin of the disk-like seed. The cells of this 

 layer are 20 to 25 microns in diameter, but only 50 to 60 microns long. The 

 third or outermost layer of the seed-coat is very uneven, consisting of a single 

 layer of cuboidal cells 30 or 40 microns thick, about most of the margin of 

 the seed, but of Y or 8 layers with a total thickness of 400 microns on the 

 sides of the seed. It is this greater thiclmess of the outer funicular layer 

 that makes up most of the bulge on the flat side of the disk-like seed. 



The whole structure and condition of the seed apparently remain quite 

 unchanged year after year so long as the fruit containing it remains attached 

 to the tree. Not only are the embryos of seeds from the oldest attached 

 fruits still plump, with the stored starch intact, but a seed from such an old 

 f niit is just as capable of germinating as one from a fruit just matured. 



A very important question arising in this connection is : Why do not 

 these seeds, immersed in the moist pulp of the parent fruit and raised each 

 summer to a relatively high temperature, germinate there, without waiting 

 to escape from the fruit ? The attempt was made to germinate seeds under 

 these conditions by treating them in the manner which was found necessary 

 to secure germination outside the fruit, that is, by chipping the seed-coat. 

 Seeds with the coats cut off at one edge Avere carefully inserted in the pulp 

 of sound fruits witli every precaution not to injure the fruit more than neces- 

 sary. The wound was then sealed with vaseline and set in a moderately 

 moist chamber to induce germination. No germinations of seeds under 

 these conditions were secured. In each case, whether the injured fruit 

 dried out, decayed, or took root in the soil, the embrj^o of the inserted seed 

 after some weeks blackened and died. The explanation of this failure of 

 the seed to germinate in its oavh fruit or in the host fruit was not discovered. 

 It is, of course, possible that it may be found in the mere exclusion of oxygen 

 by the seed-coat or by the mucilaginous pulp of its OA\m or the host fruit. 

 On the other hand, it is quite possible that the osmotic or chemical character 

 of the cell-sap of the pulp surrounding the seeds is the real obstacle to 

 germination. 



During two sojourns at Tucson the attempt was made to test the common 

 report that seedlings of Opuntia fulgida do not occur in the field about 

 Tucson (Tourney, 1905, p. 360). Diligent search was made for them in 

 many groves of these trees in the months of April and May of 1912 and 

 1915, but not a single imdoubted seedling of this species was discovered. 

 Examination of large numbers of fallen fniits showed no sign of germina- 

 tion of the seeds. It then occurred to the writer that a search on a cattle 

 3 



