ROTATION IN HAIRS AND CUTICLE-CELLS. 365 



this interesting phenomenon ; all that is necessary being to take 

 a leaf from the stem (one of the older yellowish leaves being 

 preferable), and to place it with a drop of water either in the 

 aquatic box, or on a slip of glass beneath a thin glass cover. A 

 higher magnifying power is required, however, than that which 

 suffices for the examination of the rotation in Chara or Vallis- 

 neria; the Jth inch object-glass being here preferable to the Jth, 

 and the assistance of the Achromatic condenser being desirable. 

 With this amplification, the phenomenon may be best studied in 

 the single layer of marginal cells ; although, when a lower power 

 is used, it is most evident in the elongated cells forming the 

 central portion of the leaf. The number of chlorophyll granules 

 in each cell varies from three or four to upwards of fifty ; they 

 are somewhat irregular in shape, some being nearly circular 

 flattened disks, whilst others -are oval ; and they are usually from 

 l-3000th to l-5000th of an inch in diameter. When the rotation 

 is active, the greater number of these granules travel round the 

 margin of the cells, a few, however, remaining fixed in the 

 centre ; their rate of movement, though only l-40th of an inch 

 per minute, being sufficient to carry them several times round 

 the cell within that period. As in the case of the Yallisneria, 

 the motion may frequently be observed to take place in opposite 

 directions in contiguous cells. The thickness of the layer of 

 protoplasm in which the granules are carried round, is estimated 

 by Mr. Wenham at no more than l-20,000th of an inch. A 

 peculiar undulating appearance is seen in this, under certain 

 modes of illumination, which suggests the idea of ciliary action; 

 but this appearance is decidedly affirmed by Mr. Wenham to 

 be an optical illusion. It is a curious circumstance, first re- 

 marked by Dr. Branson, that the elongated cells along the mar- 

 gin of the leaf and forming the midrib, contain a large quantity 

 of silex ; as may be readily seen by polarized light, especially 

 after the leaf has been boiled for a few minutes in equal parts of 

 nitric acid and water, which removes part of the vegetable sub- 

 stance, and thus renders the siliceous portion more distinct, 

 without destroying the form of the leaf. 1 



226. The phenomena of "rotation," however, is by no means 

 restricted to submerged Plants ; for it has been witnessed by 

 numerous observers in so great a variety of other species, that it 

 may fairly be presumed to be universal. It is especially ob- 

 servable in the hairs of the epidermic surface ; and according to 

 Mr. Wenham, who has recently given much attention to this 

 subject, "the difficulty is to find the exceptions, for hairs taken 

 alike from the loftiest Elm of the forest, to the humblest weed 

 that we trample beneath our feet, plainly exhibit this circulation." 

 Such hairs are furnished by various parts of plants ; and what is 

 chiefly necessary is, that the part from which the hair is gathered 



1 See Dr. Branson in "Quart. Journ. of Microsc. Science," vol. ii, p. 131, and vol. iii, 

 p. 374; and Mr. Wenham, Op. cit. vol. iii, p. 277. 



