SECT. III. GEMS. 407 



define, calling it the pith, loosely speaking ; though 

 in the case of woody plants it is plain that he means 

 by it the alburnum, as he says all buds are rooted 

 in that substance which is under the inner bark, 

 and form with it one body. 



Secondly, the gem is distinguished from the seed Second 

 as consisting chiefly of a pith, and having no divi- 

 sions internally into distinct parts, similar to the 

 albumen vitellus and cotyledons of the seed ; nor 

 any proper integuments externally, similar to the 

 Testa and Subtesta of the seed ; but merely a bark 

 modifying the medullary substance, and giving it 

 its external and proper form : which bark consists 

 either of the indurated substance of the granule 

 itself; or of divisions of the cellular tissue adhering 

 to it, as in the Propago ; or of the bark of the 

 parent plant by which the granule has been en- 

 veloped, as in the Gongylus ; or of the inner bark 

 forming a permanent part, or outer bark forming 

 only a temporary appendage, as in the case of the 

 bud and bulb. 



Thirdly, the gem is also often distinguished from Third 

 the seed by means of its appendages, the theca and in 

 involucrum, or sheath and involucre ; which origi- 

 nate in the bark of the parent plant. The sheath is 

 peculiar to some species of Propago, and is a vessel 

 containing a number of individuals, which it retains 

 for a certain time, and then disperses ; as may be 

 exemplified in the genus Marchantia. The invo- 

 lucre is peculiar to compound gems, it consists of 



