410 Vegetable Physiology. 



stance, to the formation of a coloured watery liquid in the same 

 sub-epidermal cells which contain the colour of red-cabbage 

 leaves ; but very often, as in the Virginia-creeper, the alteration 

 gradually affects the cells all through the leaf. The tinted liquid 

 appears to be formed at the expense of the remains of the chlo- 

 rophyll-granules, since these are found in small number in cells 

 in process of change, gradually losing their colour, and then 

 vanishing, until the cell is filled with a coloured fluid, which 

 may be extracted either by water or alcohol, but most readily by 

 the latter. So far as we could ascertain the point, the albuminous 

 lining of cell ultimately disappears here, leaving the coloured 

 liquid in direct contact with the cellulose membrane. The 

 brown colour which the fallen leaves subsequently assume, and 

 which many leaves acquire in the first instance, without dis- 

 playing red or yellow tints, arises from the transformation of the 

 cell-membranes into humus, the first step of their decay into 

 vegetable mould. 



The leaves of other trees turn almost white when they are 

 about to fall. This depends upon their cells assuming a condi- 

 tion similar to that which is constant in the white patches of 

 *' variegated" leaves. The chlorophyll and other contents vanish, 

 leaving scarcely anything but the empty cell-membrane behind. 

 The white patches and spots of variegated leaves are well known 

 to depend on the absence of chlorophyll in the subjacent paren- 

 chyma, and they therefore constitute a disease. The common 

 Aucuba, or " spotted-laurel " of our shrubberies, is a diseased 

 plant of this kind, having perfectly green leaves in Japan. The 

 disease is hereditary here because it is always grown from layers, 

 and not by seed. 



Tlie colouring-matters of fruits are similar to those of autumn 

 leaves in regard to their replacing chlorophyll previously exist- 

 ing in the same cells ; but we observe here greater variety of 

 colouring, resembling what occurs in the coloured organs of 

 flowers. 



The subject of the colouring matters producible from vegetable 

 structures, such as indigo, madder, «&c., is beyond the scope of 

 the present paper, but, when further investigated, will probably 

 throw much light upon the chemistry of vegetation. 



The aromatic or essential oils which are found more or less 

 abundantly in a great variety of plants ai'e mostly formed in 

 special cells, occurring singly or grouped into the so-called 

 " glands," in connexion with the epidermal structures. The 

 cells containing these oils are, when fully developed, filled with 

 the secretion, which is separated from the cellulose wall by a 

 thin layer of protoplasm (primordial utricle). When observed 

 in younger stages it is perceived that the oil-cells are originally 



