772 PHYSIOLOGY. 



cause of the downward descent of the root is due to the greater 

 amount of moisture which it receives in that direction, and such 

 a supply is necessary for the multiplication of its cells near the 

 extremity. 



Differences between the Get'mination of Dicotyledonous and 

 Monocofi/tedonous Seeds. — There are certain differences between 

 the germination of Monocotyledonous and Dicotyledonous em- 

 bryos, which have already been alluded to briefly (see pp. 126 

 and 127), but which require some further notice. 



1. Monocotyledonous Germination. — The seeds of Monocotyle- 

 donous plants, in by far the majority of instances, contain 

 albumen, which, as the embryo developes, is either entirely ab- 

 sorbed, or a more or less considerable portion remains, as in the 

 seed of tlie Phytelephas or Ivory Palm, where the contents of 

 the constituent cells are removed, and the walls left as a kind of 

 irkeleton. 



The single cotyledon of Monocotyledonous seeds, when they 

 contain albumen, always remain entirely (Jig. 742, c), or par- 

 tially within the integuments, during germination. The intra- 

 seminal portion of the cotyledon coiTcsponds to the hmb of 

 the cotyledonary leaf, and the portion which elongates beyond 

 the integuments (extra- seminal) re])resents the petiolar portion. 

 The latter part varies much in k^ngth, and is commonly ter- 

 minated by a sheath, which encloses the young axis with the 

 plumule. At otlicr times, there is no evident petiolar part, but 

 the sheathing portion enveloping the axis remains sessile on 

 the outside of the seed, and elongates in a tangential direction 

 to it. 



In the Oat (fig. 742), the cotyledon, c, is seen within the seed, 

 and the plumule, g, rises upwards from its axil, into the air. 



In some few Monocotyledonous Orders, such as Naiadaceae, 

 Alismacea?, &c., where the seeds are exalbuminous, the cotyle- 

 don is commonly freed from the integuments, and raised up- 

 wards with tlie plumule. 



As already noticed (p. 127), in the germination of Mono- 

 cotyledonous embryos, the radicle is not itself continued 

 downwards so as to form the root, but it gives off one or more 

 branches of nearly equal size, which separately pierce its 

 extremity, and become the rootlets (fig. 742, /). Each of these 

 rootlets, at the point wliere it pierces the radicular extremity, 

 is surrounded by a cellular sheath termed the coleorhiza (fig. 

 742, co). This mode of germination is commonly termed 

 endorhizal. 



Dicutyledonnns Germination. — The seeds of Dicotyledonous 

 plants arc cither albuminous or exalbuminous, and their germi- 

 nation in such respects, as a general rule, presents no peculiarity 

 worth notice. The two cotyledons cither remain within the 

 integuments of the seed as fleshy lobes, as in the llorse-chcstnut, 



