SEEDS, THEIK STEUCTUEE AND FUNCTIONS. 



59 



The episperm is not always membranous, but frequently so hardened as to be 

 quite woody ; in other cases it is leathery, crustaceous, bony, spongy, or fleshy. 

 Its surface is often beautifully marked, and many seeds are, from this cause, 

 interesting microscopic objects. In some seeds the episperm is expanded at the 

 edge in the form of wing-like appendages, as in the Calampelis scaler (Eccre- 

 mocarpus), and other plants of the Bignonia family ; in the Mahogany, and in many 

 of the Lily tribe. In some cases it is covered with hairs, as in the Willow-herb 

 {Epilobium), the Cotton-plant, and the different species of Asclepias. These appen- 

 dages must not, however, be confounded with those attached to many seed-vessels, 

 as, for instance, the pappus crowning the fruit of most plants of the order Composite ; 

 the wings of the Sycamore-£e#s, as they are popularly termed, and those of the 

 Ash and Dock. Upon most seeds of any magnitude, a scar is visible at the base, 

 and usually of a different colour to the rest of the integuments. This spot is called 

 the hilum or umbilicus, and marks the point by which the seed was attached to the 

 interior of the ovary or seed vessel. In the Chestnut it is black and very large, 

 covering one-fourth of the surface. In the Nemophilas, and in the Garden Pea, 

 it is white; and in some seeds it is so minute, that it is recognised with difficulty. 

 r c s The nucleus contained within the integuments, consist either of 



albumen and the embryo, as in the Primrose {Jig. I) and Potatoe 

 {fig. 2), when it is termed albuminous ; or of the embryo only, as in 

 the plants of the orders Eosacese, Cruciferae, and Leguminosse, when 

 it is called exalbuminous. In seeds of the former class, the propor- 

 tion of albumen varies greatly ; in some instances it forms the great 

 bulk of the seed, as in the Eirthwort {Jig. 3), where the embryo is 

 very minute, and in the Cocoa-nut. It may either entirely sur- a 

 round the embryo (as in^. 1), or occupy the centre of the nucleus 

 as in the Marvel of Peru {Jig. 4.) The albumen varies much in its 



* nature and consistence ; it may be mealy, consisting chiefly of cells 



a X3^^ filled with starch ' as ^ the Buck-wheat {Jig. 5), and in the Cereals, 



« -3^^ and many other plants ; or horny, as in the Coffee ; fleshy and oily, as 



fa 3 . in the Poppy and Castor-oil seeds ; and occasionally so hard as to be 



susceptible of a polish, as in the Vegetable Ivory [Phytelephas), so 



much employed for small articles of turnery 



The embryo consists of three distinct parts : the cotyledons, or a 

 seminal leaves, the plumule, or gemmule, and the radicle. It varies, s ' 

 however, in its structure in different classes of plants ; and upon a 

 these differences are founded the three great divisions of the Vege- »• 

 table Kingdom. Plants having two cotyledons, as in the Oak, and 

 the great majority of plants, are termed Dicotyledonous ; those with 

 one only, as in the Palms, Grasses, and most plants having leaves fis, *• 



