300 MONOCOTYLEDONOUS GERMINATION. 



intermediate between that impressed by gravitation and by the centri- 

 fugal force, viz., down wards and outwards ; while the stems were 

 inclined upwards and inwards. In the latter, where the force of gravi- 

 tation was neutralized by the constant change of position, the centri- 

 fugal force acted alone, by which the roots were directed outwards, at 

 the same time that the stem grew inwards. To explain these results, 

 there must be allowed 1. A more or less liquid condition of the new 

 parts of the young plant. 2. A different density in the different parts 

 of the latter. 3. A tendency of the denser parts of new plants, during 

 germination, towards the root. On the vertical wheel, the parts of the 

 young plants submitted to the centrifugal force only, had their roots 

 or densest parts at the circumference. On the horizontal, the effect 

 was intermediate between centrifugal force and gravity. The upper 

 side of leaves is under the influence of light in a marked degree, 

 for, when placed in the reverse position by the turning of a branch, 

 they twist round so as to resume their natural exposure. During 

 darkness, on the contrary, many leaves fold in such a Avay that their 

 lower surface is exposed. Some plants grow indifferently in all direc- 

 tions at the period of germination. The Misletoe and other parasites 

 direct their radicles towards the centre of the tree or plants to which 

 they are attached, while the plumule grows perpendicularly to the 

 surface. 



627. Monocoiyiedonons Germination. In Monocotyledons, there is 

 generally a perisperm present, often in large quantity, and in them 

 the cotyledon remains more or less within the seed at the period of 

 germination. The intra-seminal portion of the cotyledons, as in Canna, 

 and especially in the Coco-nut, becomes developed as a pale cellular 

 mass, which increases much, and absorbs the nutriment required for 

 the embryo. In some Monocotyledons the perisperm disappears en- 

 tirely ; in others, as in the Phytelephas or Ivory Palm, while certain 

 soluble matters are removed, the perisperm still retains its original 

 form. The intra-seminal part may be said to correspond to the limb 

 or lamina of the cotyledonary leaf. The extra-seminal portion, corre- 

 sponding to the petiole, becomes often much elongated, as in the 

 double Coco-nut, and ends hi a sheath which envelopes the axis or 

 cauliculus, and the plumule. Sometimes, however, there is no marked 

 elongation of the cotyledon, the sheath being at once formed on the 

 outside of the seed, so that the plumule and radicle are, as it were, 

 sessile on its surface. These phenomena are well seen in Canna 

 indica (fig. 527), where e is the envelope of the seed ; p the perisperm 

 or albumen; c the intra-seminal portion of the cotyledon, which 

 absorbs the nourishment ; p c the petiolary or extra-seminal portion 

 of the cotyledon, which varies in length, and may be wanting ; v the 

 sheathing portion of the cotyledon, from a slit in which, f, the plumule, 

 g, protrudes, supported on the axis or cauliculus, t ; while the radicles, 



