86 CARNEGIE INSTITUTION OF WASHINGTON. 



lution which takes place in dayUght or that which occurs with higher 

 temperatures after the optimum has been reached. 



Fruit Development in the Cactacece, by D. S. Johnson. 



The fruits of the opuntias, or flower-bearing joints as they might 

 be called, exhibit many intermediate types of structure between that 

 of typical fruits and that of ordinary vegetative joints. In external 

 form the ripe fruit may be nearly globular, with mamillse only 

 slightly evident, or it may be elongated to three or four times its 

 diameter and have, except for the cup in the torus, very nearly the 

 appearance of a vegetative joint. Internally there maybe an ovarian 

 cavity of three-fifths the diameter of the fruit, or it may be less than 

 one-fifth this diameter. The cavity may be central in the more 

 globular fruits or close to the apical end in the elongated ones. Over 

 half of these mature fruits contain a number of ripe seeds, which 

 may vary from 1 to 100 to each fruit. Other fully grown fruits 

 contain only shriveled rudiments of ovules. Some of these evidently 

 ceased development at about the time of poUination, while others 

 had attained to one-fourth or half the mature size when degeneration 

 set in. Not all flowers, however, give rise to structures having the 

 form of mature fruits. A certain percentage of the potential fruits 

 drop off soon after flowering. One is inclined to conclude that pol- 

 lination occurs only in mature fruits, and also that among these 

 fruits seeds are formed only in those cases where fertilization as well 

 as pollination takes place. The latter conclusion is rendered doubt- 

 ful, however, by the fact noted above, that matured fruits often 

 contain seeds that have degenerated after becoming half grown. 

 Field work is needed to settle both these questions definitely. 



In the fruits increase in age is marked not only by increase in size 

 and change in the form of the fruit as a whole, but also by changes 

 in the size and character of the areolae. The latter are but 1 or 2 mm. 

 across when the subtending leaf falls off. In older fruits they pro- 

 trude far above the surface and become 5 or 6 mm. in diameter. 

 This growth is due to the development of some dozens of new bristles 

 and of successive ring-like series of new hairs. The other organ rudi- 

 ments present in the areola apparently remain practically dormant. 



The matured fallen fruits have the unique function, for a fruit, 

 of serving as propagules for the development of new plants. The 

 first indication of the origin of a new plantlet from such a fruit is the 

 pushing out of one or several young roots, which form the apical 

 margin of an areola. It is worthy of note that these roots arise above 

 the clump of bristles, beyond the limits of the growing-point of the 

 axillary bud, and not within these, as is always true of the flower 

 rudiments, and apparently of the new shoot rudiments also. These 

 adventitious roots are probably entirely internal in origin. At any 

 rate, vascular supply of the plantlet is connected quite independently 



