284 THE MICROSCOPICAL NEWS. 
is called, are sugar, oil and fat, and starch. When assimilation is 
in excess of the present demands of the plant, the excess of 
nutrient matter is removed from the cells in which it is formed, 
and stored up in other parts for future use. When the first-formed 
products are soluble in the cell sap, their transmission is of course 
readily explained by the constant circulation of sap, but when in- 
soluble the transmission must be otherwise accounted for. The 
substance, in fact, must undergo a process of meéastasis, or con- 
version into some soluble body, and this, on its arrival at the place 
of deposition, may be reconverted into its original insoluble state, 
and so remain until it is required for use. 
Of these substances starch is by far the most important. | It is 
one of a class of bodies called carbo-hydrates, that is, it consists of 
a compound of carbon with oxygen and hydrogen in the proportion 
of x atom of oxygen to 2 of hydrogen, or in other words, in the 
proportion in which they-exist in water. So we may regard starch 
as a compound of carbon and water. ‘This fact should be borne 
in mind in connection with its use as a dietetic, for its hydrogen 
being already completely oxidised or burnt cannot contribute to 
the production of heat in the body, and so does not take equal 
rank as a heat producer with the fats and oils in which the hydro- 
gen as well as the carbon is in excess, Starch is insoluble in water 
and cell sap. When analysed it yields the proportional constitution 
C,H,,0O;, but these figures do not represent the true complexity 
of its molecule, which is best represented by xC,H,,O;, in which 
z is used in the algebraical sense to stand for an unknown quantity, 
but probably = 3, so that the true formula would be C,,H3,9Q0},;. 
It is closely related to gZycogen or liver starch, dextrin and cellulose, 
and although all these bodies differ in their physical and many of 
their chemical properties, they have exactly the same composition, 
or, as chemists say—they are zsomeric. From glucose or grape 
sugar, the formula of which is 7C,H,.0,. It differs only in the 
possession of one molecule less of water, and is easily convertible 
into that substance. Treated with a very dilute solution of zodine 
starch assumes a purplish blue colour, and this reaction being very 
delicate and highly characteristic of starch, affords the most 
valuable test for that substance, even the optical one to be pre- 
sently mentioned not excepted. It has been usually assumed that 
the iodine formed a definite compound with the starch (the so-called 
iodide of starch), but there is reason to believe that it is only 
deposited on the starch in a metallic state. 
By the action of an organic nitrogenous body of very doubtful 
composition, called diastase, produced in germinating seeds, and 
by other means, starch is converted into dextrin and glucose, both 
of which are readily soluble in water or the cell sap, This, of 
course, is important in explaining how the insoluble starch may 
