354 MONOCOTYLEDONOUS GERMINATION. 



parallel to the axis of rotation. Kniglit* put Mustard seeds and 

 Prench beans on the circumference of two wheels, which were put in 

 rapid motion, the one in a horizontal, and the other in a vertical 

 manner ; and he found that in the former the roots took a direction 

 intermediate between that impressed by gravitation and by the centri- 

 fugal force — viz., downwards and outwards, while the stems were 

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

 gravitation was neutralised by the constant change of position, the 

 centrifugal force acted alone, by which the roots were directed out- 

 wards, at the same time that the stem grew inwards. To explain 

 these results, there must be allowed — 1. A more or less liquid con- 

 dition 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 wheel 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 way that their lower surface is exposed. Some plants grow 

 indifferently in all directions at the period of germination. The 

 Mistleto and other parasites direct their radicles towards the centre 

 of the plants to which they are attached, while the plumule grows 

 perpendicularly to the surface. 



MONOCOTYLEDONOUS Geemination. — 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 intrarseminal portion of the cotyledons, as in 

 Canna (fig. 626), and especially in the Coco-nut, becomes developed 

 as a pale cellular mass, which increases much, and absorbs the nutri- 

 ment required for the embryo. In some Monocotyledons the perisperm 

 disappears entirely ; in others, as in the Phytelephas or Ivory Palm, 

 while certain soluble matters are removed, the perisperm stOl 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, corresponding to the petiole, becomes often much elongated, 

 ■as in the double Coco-nut, and ends in 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 

 indioa (fig. 626), where e is the envelope of the seed; p, the peri- 



* See Knight's Horticultural Papers, London, 1841, p. 124. 



