CHAP. U. FRUIT. 189 



In septifragal dehiscence the dissepiments 

 adhere to the axis and separate from the 

 valves, as in Convolvulus ; or in the dia- 

 gram 170., lettered as before. 



In sutured dehiscence there are no disse- 

 piments, the fruit being composed of only ^ 

 one carpel, as the pea. 



Besides these regular forms of valvular dehiscence, there is 

 a mode which obtains in a very few plants, called circumscissile. 

 This occurs by a transverse circular separation, as in Ana- 

 gallis ; in Jeffersonia it only takes place half round the fruit. 



Valvular dehiscence, which is by far the most common 

 mode by which pericarps open, must not be confounded wdth 

 either rupturing or solubility, — irregular and unusual con- 

 trivances of nature for facilitating the dispersion of seeds. In 

 valvular dehiscence the openings have a certain reference to 

 the cells, as has been already shown ; but neither rupturing 

 nor solubility bear any distinct relation to the cells. Rup- 

 turing consists in a spontaneous contraction of a portion of the 

 pericarp, by which its texture is broken through, and holes 

 formed, as in Antirrhinum and Campanula. Soluhility arises 

 from the presence of certain transverse contractions of a one- 

 celled pericarp, through which it fmally separates into several 

 closed portions, as in Ornithopus. 



For the nature of the placenta and umbilical cord see the 

 observations under ovary. These parts, which are mere 

 modifications of each other, essentially appertain to the peri- 

 carp, in which the former often acquires a spongy dilated sub- 

 stance, occasionally dividing the cells by spurious dissepi- 

 ments, and often giving to the fruit an appearance much at 

 variance with its true nature. In some seeds, as Euonymus 

 Europaeus, it becomes exceedingly dilated around each seed, 

 forming an additional envelope, called aril. The true charac- 

 ter of this organ was unknown till it vvas settled by Richard : 

 before his time the term was applied, not only in its true sense 

 to an enlargement of the placenta, but also to the endocarp of 

 certain Rubiaceas and Rutaceae, to the seed coat of Jasniinum, 

 Orchideae, and others, and even to the perianth of Carex. A 

 very remarkable instance of the aril is to be found in the nutmeg, 



