854 . MONOCOTYLEDONOUS GERMINATION, 
seed ; the cotyledonary portion accordingly perishes, and the 
act of germination is complete. 
Direction of Plumule and Radicle.—The cause which leads to 
the development of the axis of the embryo in two opposite di- 
rections has not yet been satisfactorily demonstrated, although 
much has been written on the subject. By some it has been 
referred to the action of darkness and moisture on the root, and 
that of light and dryness on the stem. By others it has been 
attributed to gravitation and the state of the tissues; and 
others, again, have regarded osmotic action as the cause. All 
these explanations are unsatisfactory, and need not be further 
alluded to. Darkness has been shown to have no influence on 
the direction of the root, which is probably determined by the 
greater amount of moisture usually met with in the soil, and by 
gravitation or geotropism (see page 862). In Trapa natans the 
radicle is directed upwards towards the surface of the water in 
which the plant grows. 
Differences between the Germination of Dicotyledonous and 
Monocotyledonous Seeds.—There are certain differences between 
the germination of Monocotyledonous and Dicotyledonous em- 
bryos, which have already been alluded to briefly (see page 134), 
but which require some further notice. 
1. Monocotyledonous Germination. —The seeds of Monoco- 
tyledons, in by far the majority of instances, contain albumen. 
This, as the embryo developes, is usually entirely absorbed ; but 
sometimes, asin theseed of Phytelephas, the contents of the consti- 
tuent cells are removed, and the walls left as a kind of skeleton. 
The single cotyledon of Monocotyledonous seeds, when they 
contain albumen, always remains entirely (fig. 765, c), or partially 
within the integuments, during germination. In the latter case, 
the intra-seminal portion of the cotyledon corresponds to the 
limb of the cotyledonary leaf, and the portion which elongates 
beyond the integuments (extra-seminal) represents the petiolar 
portion. The latter part varies much in length, and is commonly 
terminated by a sheath, which encloses the young axis with the 
plumule. In the Palms this petiolar portion is often several 
inches in length. At other 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, as in the Oat (fig. 765), where the cotyledon, c, 
remains within the seed, and the plumule, g, rises upwards from 
its axil into the air. 
In some few Monocotyledonous orders, such as Naiadacez, 
Alismaceze, &c., where the seeds are exalbuminous, the coty- 
ledon is commonly freed from the integuments, and raised 
upwards with the plumule. 
As already noticed (page 134), in the germination of Mono- 
cotyledonous embryos, e.g. the Grasses, the radicle is not itself, 
except in rare cases, continued downwards so as to form the 
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