348 



ORGANOGRAPHY. 



In by far the majority of cases the two cotyledons are nearly 

 of equal size, as in the Pea {fig. 160, c, c), but in some embryos, as 

 in Trapa, some Hirceas, &c. (fig. 747, cc), they are very unequal 

 Again, while the cotyledons usually form the greater part of the 

 embryo, in other instances, as in Pekea butyrosa {fig. 749, c), they 

 form but a small portion. In the Carapa {fig. 748) again, the 

 two cotyledons become united more or less completely into one 

 body, so that the embryo appears to be monocotyledonous ; 

 but it is readily distinguished by the different position of the 

 plumule in the two cases; thus in the monocotyledonous embryo, 

 the plumule is situated just below the surface, but here the 

 plumule, g, is in the axis of the cotyledons. 



The cotyledons are sometimes altogether absent, as in Cuscuta. 

 At other times the number is increased, and this may either 

 occur as an iiTcgular character, or as a regular condition, as in 

 many Coniferai {fig. 750, c), where we frequently find six, nine, 

 or even fifteen cotyledons; hence such embryos have been termed 

 polycotyledonous. It seems probable that this appearance of a 

 larger number of cotyledons than is usual in Dicotyledonous 

 Plants, arises from the normal number becoming divided down 

 to their bases into segments. In all cases where the number of 

 cotyledons is thus increased, they are arranged in a whorl {fig. 

 750). The cotyledons are usually thick and fleshy, as those of 

 the Bean and Almond {fig. 746), in Mhich case they are termed 

 fleshy ; at other times they arc thin and leaf-like, as in the Lime 

 (fig. 741), Euonymiis, &c., when they are said to be foliaceous. 

 The foliaceous cotyledons are frequently provided with veins, and 

 stomata may be also sometimes observed on their epidermis; 

 these parts are rarely to be found in fleshy cotyledons. Flesliy 



cotyledons serve a similar 

 purpose to the albumen, by 

 acting as a reservoir of nutri- 

 tious matters for the use of 

 the young plant during ger- 

 mination ; licnce, when the 

 albumen is absent, the coty- 

 ledons are generally propor- 

 tionately increased. 



The cotyledons are com- 

 monly sessile, and their mar- 

 gins are usually entire, but 

 exceptions occur to both these 

 characters ; thus in Geranium 

 mollc {fig.lbX), they are petio- 

 late; wliile in the Lime {fig. 

 741), Walnut, and Geranium, {fig. 751) they arc lobed. 



The cotyledons also vary in their relative positions to each 

 other. Generally they arc placed parallel, or face to face, as 



Fig. 750. 



Fig. 751. 



tQS 



Fig. TSO. Polycotyle- 

 donous embryo of a 

 I'infut beginning to 



ferminatc. r. Coty- 

 edons. r. Radicle. 



t. Tigelle Fifi.ir,\. 



The embryo of (,'era- 

 niiim molle. c. Coty- 

 ledons, each of which 

 is furnighed with a pe- 

 tiole, p. r. Radicle. 



