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POPULAR SCIENCE REVIEW. 
imbedded in the protoplasm in the form of distinct grains, the 
form of which is nearly uniform in the same species of plant. 
If a drop of dilute solution of iodine is placed on a thin section 
of a potato, the grains of starch are beautifully brought out 
under the microscope by the bright violet colour they assume, 
the cell-wall remaining colourless. The pith of the elder or 
the bulb-scales of the hyacinth form equally good prepara- 
tions, starch being invariably found in those parts of the plant — 
as bulbs, tubers, rhizomes, albuminous seeds, &c. — where the 
food-material is stored up for the nutrition of the young plant, 
whether it be developed from an embryo or a leaf-bud. Starch- 
grains generally contain a body which may be defined as the 
nucleus, and the growth of the grain invariably takes place by 
intussusception, new particles of the formative material becoming 
intercalated, according to Sachs, between those already in ex- 
istence ; the new matter assuming the form of concentric layers 
of greater and less density, that is, containing a smaller and 
larger admixture of water. Grains sometimes become com- 
pound by the production of several nuclei in their interior. 
Chlorophyll is the substance which gives the green colour to 
leaves and branches, and may be considered as a peculiar form 
of protoplasm, the green colour being probably due to the 
admixture of a very small quantity of iron. The constitution 
of this substance has been very carefully investigated by Mr. 
H. C. Sorby, who finds it to be composed of different bodies, 
especially a blue and a yellowish-green one, the mixture of 
which gives the familiar leaf-green colour. The chlorophyll 
always occurs in the form of minute granules interspersed 
through the protoplasm, which vary their position in the cell 
according to the intensity and direction of the light. They 
are formed only in the presence of sunlight, either direct or 
diffused ; and it is only those parts of the plant which possess 
them that have the power of decomposing the carbonic acid of the 
atmosphere. Independently of this alteration in position of the 
chlorophyll-grains, the protoplasm itself has often — perhaps 
always — a rotating or circulating motion within the cell ; this 
can be easily perceived in the leaves of Vallisneria , the stems of 
Chara , the hairs on the filaments of Tradescantia , the stinging- 
hairs of the nettle, &c. 
Raphides are minute crystals, generally of oxalate of lime, 
contained within the cell; their purpose is not known; possibly 
the fixation of oxalic acid, which might otherwise be injurious 
to the plant. They abound in many plants, as the stem of 
the Cactus , the leaf-stalk of rhubarb, &c., and may very easily 
be made out in a section of the leaf of the hyacinth. 
Having described the structure of the cell and its contents, 
we may now follow the various modes in which cells multiply, 
