58 



EMBRYO. 



[chap. 



DractCna, and other plants just examined, the seeds arc free 

 from the pericarp, as they are also in l^icotyledons generally ; 

 the adhesion, in this case, may be regarded as accidental, 

 though it is very characteristic of the fruit of grasses. 

 Cutting the grain open, we find the embryo near the base 

 occupying about one-fourth or one-fifth of its contents, the 

 rest of the seed being filled with a starchy albumen. 



8. The structure of this embryo we must endeavour to 

 understand, though in order to make it clearly out very 

 careful sections must be made 

 through it lengthwise. The accom- 

 panyiDg cut will supply a good idea 

 of the arrangement of its parts. We 

 do not find the first leaves of the 

 embryo opposite to each other, 

 forming a pair of cotyledons, as in 



Fig. 42. Longitudinal sections, 1 1 t^ • i i 



cut at right angles, of the the Bean and Other Dicotyledons, 



Embryo of Wheat, showing 



the cotyledon c, the plumule but they are alternate; the outer- 



//, and the root-buds, r. . ■, ^ • 1 -i 1 



most only being regarded as a seed- 

 leaf or cotyledon. Those which it sheaths belong to the 

 plumule. The cotyledon being single, the embryo of Wheat 

 is called monocotyledonous. The lower part of the embryo is 

 the radicle. This never directly elongates in germination, 

 but the internal, rudimentary root-buds, r, burst through it 

 and develope into the root-fibres of the plant. The pro- 

 cess of germination is similar to that of Dicotyledons, 

 with this difference in regard to the origin of the root : the 

 sheathing portion of the cotyledon is protruded from the 

 seed, and embraces the base of the plumule, which 

 ultimately developes into a stem. 



Now a structure similar to that of Wheat we find in the 

 seeds of other corn-plants. In Rice the grain (fruit) presents 

 a ditferent appearance, owing to the circumstance that in 



