66 THE DATA OP BIOLOGY. 



up, to be subsequently modified for divers purposes, starch is 

 the most important of these; and the process of storage is 

 suggested by the structure of the starch-grain. This con- 

 sists of superposed layers, implying intermittent deposits: 

 the probability being that the variations of light and heat 

 accompanying day and night are associated now with arrest 

 of the deposit and now with recommencement of it. Like 

 in composition as this stored-up starch is with sugar of one 

 or other kind, and capable of being deposited from sugar and 

 again assuming the sugar form, this substance passes, by 

 further metabolism, here into the cellulose which envelopes 

 each of the multitudinous units of protoplasm, there into the 

 spiral fibres, annuli, or fenestrated tubes which, in early stages 

 of tissue-growth, form channels for the sap, and elsewhere 

 into other components of the general structure. The many 

 changes implied are effected in various ways : now by that 

 simple re-arrangement of components known as isomeric 

 change; now by that taking from a compound one of its ele- 

 ments and inserting one of another kind, which is known as 

 substitution ; and now by oxidation, as when the oxy-cellulose 

 which constitutes wood-fibre, is produced. 



Besides elaborating building materials, the protoplasm 

 elaborates itself that is, elaborates more of itself. It is 

 chemically distinguished from the building materials by the 

 presence of nitrogen. Derived from atmospheric ammonia, 

 or from decaying or excreted organic matter, or from the 

 products of certain fungi and microbes at its roots, the nitro- 

 gen in one or other combination is brought into a plant by 

 the upward current; and by some unknown process (not 

 dependent on light, since it goes on equally well if not better 

 in darkness) the protoplasm dissociates and appropriates 

 this combined nitrogen and unites it with a carbo-hydrate 

 to form one or other proteid albumen, gluten, or some 

 isomer; appropriating at the same time from certain of the 

 earth-salts the requisite amount of sulphur and in some cases 

 phosphorus. The ultimate step, as we must suppose, is the 



