THE FEUIT OP OPTJNTIA FULGIDA. 29 



THE PERENNATING FRUIT: ITS STRUCTURE AND SECONDARY GROWTH. 



The matured fruits of Opuniia fulgida not only do not ripen at the end of 

 the growing-season, but (as noted above) they are neither shed from the 

 plants as most fruits are, nor do they open in any way to discharge their 

 seeds. On the contrary, these fruits remain year after year, attached and 

 actively growing, im.til they may become 40 or 50 mm. in diameter and 70 

 or 80 mm . long. The growth of these fruits in length must be chiefly 

 primary ; that is, each fruit attains practically its maximum length during 

 its first season's growth. Thus the longer fruits mentioned must have been 

 exceptionally long at the start. 



Growth in thickness of the larger fruits is, on the contrary, largely 

 secondary. It is accomplished by the persistent activity of the fascicular 

 cambium and by the general multiplication of the parenchyma cells between 

 the bundles. Whether there is an increase in the central medullary 

 parenchyma of the fruit has not been determined with certainty, but 

 there appears to be some little increase in the interior diameter of the 

 vascular ring. The cortical parenchyma, inside the hypoderm, may in- 

 crease in thickness from about 2 mm. in the just-matured fruit to 5 mm. 

 or more in a fruit several years of age. This radial growth seems to be 

 aliiiost entirely a result of the radial elongation of the cortical cells, as the 

 number of these along a radius ranges from 20 to 25 in both the just-matured 

 fruit and in that 3 or 4 years old. 



The fascicular cambium of the perennating fruit gives rise to numerous 

 phloem and xylem elements, which increase the radial dimension of the 

 bundle from 0.7 or 0.8 mm. in the newly matured fruit to 3 or 3.5 mm. in a 

 fruit several years old. The radial extent of the xylem in the newly formed 

 fruit is about 0.6 mm. and that of the phloem 0.2 mm., while in an older 

 fruit the dimensions are 2.6 mm. for the xylem and 0.6 mm. for the phloem. 

 This shows that the growth of the two tissues has kept their bulk much the 

 same relatively. The tangential growth of the bundles is relatively slight, 

 the outer edge of a bundle 4 or 5 years old being only half as wide again as 

 that of one in the first-year fruit. ISTo definite annual rings could be dis- 

 tinguished in the wood of these older fruits. The number of larger xylem 

 elements along a single radius in a young fruit is about 25, while in a 4 or 5 

 year fruit there may be 80 or 90 vessels on a single radius. 



The parenchyma of the pith-rays differs from that of the cortex by show- 

 ing a vigorous cell multiplication. The pith-ray of the just-matured fruit 

 may have but 8 or 10 cells in the radial extent of the bundle, while in a 

 fruit 4 or 5 years old a pith-ray may be made up of 25 or 30 cells in the 

 radial width of the bundle. This growth is apparently due to the general 

 division of cells throughout the ray, as no trace of an interfascicular 

 cambium is discoverable. 



Two structures only on the surface of the fruit undergo important changes 

 as the fruit ages. These are the corky periderm and the areoles. At the 



