IV.] 



DICOTYLEDONS. 



45 



sisting of two large thin cotyledons, and a radicle, enclosed in 

 a quantity of a uniform, white, firm substance, resembling the 

 cotyledons of the Pea in texture. This substance, in which 

 the embryo is embedded, is called the albiimen of the seed. 

 It is at the expense of this albumen that the embryo is 

 enabled, during its germination, to develope a root and stem. 

 The albumen in the seed of Castor-oil substitutes the store 

 of nutrient matter contained in the thick cotyledons of Pea> 

 the embryo alone of which, with the testa, constitutes the 

 entire seed. Seeds containing, besides an embryo, a deposit 

 of albumen, whether large or small, are said to be albu- 

 minous. Seeds like those of the Pea and Orange, which 

 contain an embryo only, are exalbuminous. Between the 



Fig 31 Vertical sections ot seed of Castor-oil plant {Ricinm communis) ; to the 

 left cut in the plane of the cotyledons, to the right at right angles to the 

 cotyledons, showing the copious albumen in which the embryo is embedded. 



two extremes of abundant albumen (Castor-oil plant) and no 

 albumen at all (Pea, Orange) we have every degree. 



15. Like the Pea, therefore, the Castor-oil plant is dicoty- 

 ledonous ; and as the character expressed by this term (the 

 possession of a pair of cotyledons) is common to plants with 

 irregularly net-veined leaves, and with the parts of their 

 flowers in fours or fives (with but a comparatively small 

 number of exceptions), botanists employ the term dicoty- 

 ledons as the name of a great Class of flowering plants, 

 including all those which present the above characters. 



