176 MINUTE STRUCTURE OF THE FRUIT. 



THE FRUIT. 



493. The fruit is the ripened pistil. But, as shown in Vol- 

 ume I., "it is a loose and multifarious term, applicable alike to 

 a matured ovaiy, to a cluster of such ovaries, at least when 

 somewhat coherent, to a ripened ovary with calyx and other 

 floral parts adnate to it, and even to a ripened inflorescence when 

 the parts are consolidated or compacted." 



494. Histologically considered, fruits present few difficulties, 

 although the changes in form which a pistil undergoes as it ripens 

 are not greater than the changes which it may suffer in minute 

 structure. These histological changes are referable to a few 

 simple kinds : (1) a great development of sclerotic elements, seen 

 in the harder dry-fruits and in the putamen of all stone-fruits ; 

 (2) a large increase in the amount of soft-walled parenchyma, 

 containing sap, as in the pulp of all fleshy fruits; (3) a consid- 

 erable development of color, especially in the superficial parts. 



495. Sections to exhibit the structure of the very hard parts 

 of fruits are made most easily b} T carefully grinding the parts 

 on a fine oil-stone. First, a fragment of the hard shell of a nut 

 or of the putamen of a drupe is obtained by means of an}' strong 

 cutting instrument, and a flat surface parallel to the plane of 

 the section desired made by a clean file. On a glass slide a 

 drop of Canada balsam is placed, and heated until the more 

 volatile portion is expelled (see 111). Then the flat side of the 

 object just prepared is held upon this balsam until the latter 

 becomes cool and hard ; and when thus securely fastened, the 

 specimen is rubbed down on an oil-stone to any required de- 

 gree of thinness. It is removable from the slide by oil of 

 turpentine, and can afterwards be mounted in a fresh portion of 

 balsam or of benzol-balsam (see 112). 



496. The contents of the parenchyma cells of fruits depend 

 very largely on the degree of maturity of the fruit. Changes in 

 the contents go on from the formation of the fruit until it is fully 

 ripe. In some of the more common cases these consist largely 

 in the production of various sugars, especially that which is 

 known as fruit-sugar ; and organic acids, for instance, citric, 

 tartaric, and malic acids. A consideration of these changes 

 belongs to Part II. 



497. The coloring-matters in fruits, like those in flowers, are 

 either color-corpuscles (chromoplastids), or substances dissolved 

 in the cell-sap. In a few cases the walls of the cells them- 

 selves have more or less color. 



