r
the new shoot, where it is at length converted into cellulose and used
to build up plant-structure.

454. But that which builds plant-fabric is not the cellular structure
itself; the work is done by the living protoplasm which dwells within
the walls. This also has to take and to assimilate its proper food, for
its own maintenance and growth. Protoplasm assimilates, along with the
other three elements, the nitrogen of the plant's food. This comes
primarily from the vast stock in the atmosphere, but mainly through the
earth, where it is accumulated through various processes in a fertile
soil,--mainly, so far as concerns crops, from the decomposition of
former vegetables and animals. This protoplasm, which is formed at the
same time as the simpler cellulose, is essentially the same as the flesh
of animals, and the source of it. It is the common basis of vegetable
and of animal life.

455. _So plant-assimilation produces all the food and fabric of
animals._ Starch, sugar, the oils (which are, as it were, these
farinaceous matters more deoxidated), chlorophyll, and the like, and
even cellulose itself, form the food of herbivorous animals and much of
the food of man. When digested they enter into the blood, undergo
various transformations, and are at length decomposed into carbonic acid
and water, and exhaled from the lungs in respiration,--in other words,
are given back to the air by the animal as the very same materials which
the plant took from the air as its food,--are given back to the air in
the same form that they would have taken if the vegetable matter had
been left to decay where it grew, or if it had been set on fire and
burned; and with the same result, too, as to the heat,--the heat in this
case producing and maintaining the proper temperature of the animal.

456. The protoplasm and other products containing nitrogen (gluten,
legumine, etc.), and which are most accumulated in grains and seeds (for
the nourishment of their embryos when they germinate), compose the most
nutritious vegetable food consumed by animals; they form their proper
flesh and sinews, while the earthy constituents of the plant form the
earthy matter of the bones, etc. At length decomposed, in the secretions
and excretions, these nitrogenous constituents are through successive
changes finally resolved into mineral matter, into carbonic acid, water,
and ammonia or some nitrates,--into exactly or essentially the same
materials which the pla