1546 



A TEXTBOOK OF THEORETICAL BOTANY 



flowers of the umbel they are absent. Where one side of a bilateral struc- 

 ture is more developed than the other, the difference is often designated 

 by plus and minus. Thus in the fruit of Antirrhinum majus the lower 

 carpel of the bicarpellary fruit is always "plus", the upper, smaller carpel 

 is " minus ". 



Heterocarpy is almost always associated with some constant difference 

 in the positions of the two kinds of fruit on the plant, which suggests 

 differences of nutrition as a possible explanation. There are, however, 

 some exceptions, one being a climbing member of the Euphorbiaceae, 

 Tragia vohihilis from Brazil. The fruits are axillary and borne singly on 

 long pedicels. Most are the typical trimerous capsules of the family, but 

 some fruits are unicarpellary and one-seeded, the two sides of the carpel 

 growing out into long horns and the ventral suture being extended into a 

 third, shorter horn. Both types of fruit are produced indiscriminately in 

 the leaf-axils. The horned fruits are indehiscent and are probably dispersed 



by animals, while the tricarpellary fruits 

 dehisce in the usual way. 



It will have been gathered from some 

 of the foregoing observations that some 

 parts of the flower other than the gynoe- 

 cium are affected by post-fertilization 

 developments. Some of these later floral 

 developments function for the protection 

 of the fruit, others for its dispersal, some- 

 times both are aided. The calyx is an 

 organ which is often affected in this way, 

 not only persisting as the fruit ripens 

 but changing its appearance and some- 

 times its size. A familiar example is the 

 Bladder Cherry, Physalis alkekengi, the 

 synsepalous calyx of which is trans- 

 formed into a thin, orange-coloured 

 balloon (see Fig. 1130) about an inch 

 and a half across, entirely enclosing the 

 berry fruit. Similar, but uncoloured 

 calyx envelopes (Fig. 1406) occur also in 

 other Solanaceae. Some members of 

 the Convolvulaceae have enveloping 

 calyces in which the sepals become 

 fleshy and excrete water internally, the 

 so-called "water calyces", the fruit de- 

 veloping in a water bath. This doubtless 



helps to protect the young fruit from 

 Fig. 1406. — Hvoscvamus niger. , . . 1 ^ u ^u 4. ■ e „.,^ 



Henbane. Infrutescence show- desiccation, but whether it IS ot any sur- 



ing the enlarged, persistent vival value is not known. 



calyces which conceal the small ry-ii n 1 • ^u f •. ^f 



capsule fruits. The swollen sepals m the fruit of 



