554 PHYSIOLOGY. 



such as Sea- kale, Celery, forced early Rhubarb, &c. And the 

 tissues of the plants thus grown have exactly that weak, succulent 

 character which is so striking in most leafless parasitic plants and 

 Fungi. 



Further applications of these facts will be dwelt on in the succeeding 

 Sections. 



Assimilation of Inorganic matter. By far the most striking 

 phenomena of vegetative life are those in which inorganic matters 

 are assimilated, and the gaseous and liquid constituents of the 

 atmosphere and soil supply the requisite food. 



If all plants required organic food, the organized substance upon the 

 globe must continually decrease, since, as we have just seen, those which 

 do live upon organic matter waste this through decomposition by contact- 

 action. But the organic matter of soils, upon which plants grow and decay 

 in successive crops, undergoes continual increase, as we observe in the 

 accumulation of vegetable mould on undisturbed grass plains and in forests 

 where the debris (fallen leaves, underwood, &c.) is^not removed. 



Food of Plants. The majority of plants feed upon water, 

 carbonic dioxide, ammonia, nitrates (and perhaps other nitrogenous 

 compounds), with small quantities of various other elements, such 

 as sulphur, phosphorus, and the salts of lime, potash, &c. Such 

 plants can only flourish under the influence of light ; and under 

 this influence they produce, from the above materials, new cellulose 

 &c. and protoplasmic matter. The assimilation is in such cases , 

 as a general rule, accompanied by the assumption of a green colour, 

 from the formation of chlorophyll. 



Exceptions to the last assertion appear to exist in the red, olive, and 

 other peculiarly coloured Algae, in which no chlorophyll is produced ; 

 bat we are ignorant of the processes which go on in the vegetation of 

 these plants. 



Nutrition in Alg9B. The history of the changes which take place 

 in the cell-contents of the green Confervoid Algae (figs. 512, 513), 

 which we are able to observe to a certain extent beneath the micro- 

 scope, affords some material towards the comprehension of the pro- 

 cesses which have their seat in the green parts of the higher plants. 



We observe, in the elongating apical or branching cells of the Confer- 

 voids, that the contents of the nascent parts (as in the upper half of the 

 dividing-cell of (Edoaoninm &c.) are chiefly composed of colourless pro- 

 toplasm, with watery cell-sap. Under the influence of light, green 

 chlorophyll-corpuscles become more and more abundant; and, under 

 favourable circumstances of light &c. (accompanied by liberation of 

 oxygen gas), the chlorophyll-corpuscles soon present starch-granules in 

 the interior, which multiply and increase considerably in size. This 

 formation of starch occurs chiefly after the cell has attained its full 

 growth, and may be regarded as a continuation of the process which 



