60 



SEEDS, THEIR STRUCTURE AND FUNCTIONS. 



with parellel veins, are Monocotyledonous ; those without cotyledons, as in the case of 

 *" Perns, Mosses, Lichens, and Sea- weeds, are termed Acotyledonous ; 



though this term is hardly correct, as we will hereafter explain. 

 In the Dicotyledonous embryo, of which all our figures, with the 

 a exception of 10 and 11 are examples, the two cotyledons are usually 

 in close contact, the plumule being enclosed between them as in the 

 Bean {fig. 5), where one of the cotyledons has been cut away. 

 fig- 5 - They are generally entire, but in a few cases they are lobed, as in 

 the Walnut; or deeply divided, as in the Lime-tree and Fir 

 {fi9- 1)'f this last is often termed polycotyledonous, but appa- 

 rently without sufficient reason. 



The cotyledons are commonly straight (as in figs. 1, 3, and 6) ; 

 in other cases they, as well as the entire embryo, are slightly ' 

 curved (as in fig. 5) ; and in many seeds they are arranged 

 spirally (as in fig 2). In the Marvel of Peru {fig. 4), where the 

 embryo encloses the mealy albumen, it is termed peripherical. 



In the Convolvulus tribe, the cotyledons are crumpled so that fig. 6. 



when unfolded they expose a large surface ; in the Pomegranate, they are 

 i rolled up laterally, so as partially to surround each other ; and in the Gera- 

 nium family, they are plaited. 



The radicle, or point from which the root proceeds, is usually 

 straight ; but in certain important orders it is curved, and 

 gives a marked character to the seed. In the Cruciferse it is , 

 curved, so as to be parellel to the cotyledons, being either folded 

 against the edges, as in the "Woad {Isatis tinctoria, fig. 8), or on 

 ji s , 7, the back of one of them, as in the Wall-flower {fig. 9). In 

 some plants of the same order, the cotyledons are two or three times folded ; and it 

 is on these distinctions that the subdivisions of this order are founded. 



The position of the radicle is also an important mark of dis- 

 tinction. It is usually opposite the hilum (as in figs. 2, and 4) ; but 

 in all the plants of the Nettle tribe, in the Cistus, and in the Buek- 

 ^ wheat ( fig. 5), it is inverted, and is then termed superior. This is 

 almost the only point of difference between the orders Compositae and 

 Dipsacea3. In the Primrose {fig. 1), and all plants of the order 

 Priniulaceae, the embryo lies across the hilum, and the radicle is 

 therefore lateral. 



Monocotyledonous and Acotyledonous seeds, with the phenomena of germination 

 in our next. 



[The references are the same for each of the above wood-cuts : s, the spermodcrm or integument ; 

 a, the albumen ; e, the embryo ; c, the cotyledons ; r, the radicle ; g, the gemmule or plumule.] 



