WITHERED LEAVES. 209 



the plant as protecting or formative envelopes, by their form, hard- 

 ness, or power of attracting water. The nutrient substances which 

 are essential to the Hfe and growth of vegetable cells are, first, 

 those which enter into the composition of protoplasm ; and these 

 are carbon, oxygen, nitrogen, hydrogen, and sulphur, and in addi- 

 tion to these essential elements may be enumerated iron, calcium, 

 potassium, magnesium, phosphorus, sodium, and chlorine. These 

 various nutrient substances are taken up, (in the higher orders of 

 plants,) partly by the roots from the soil in the form of vapour, 

 and partly by the leaves through the agency of the stomata, in the 

 gaseous condition from the atmosphere ; for it is only in one or 

 other of these conditions that they can reach and become the 

 nutriment of the protoplasm, through the substance of the cell- 

 walls, and this is effected by the action of osmose, which is simply 

 the mixing of two fluids of different densities, separated by a per- 

 meable membrane such as is found in the cell-wall. The aqueous 

 particles absorbed from the soil by the roots are carried upwards 

 through the fibro-vascular bundles to the leaves, where the surplus 

 quantity not required in the vegetative process is exhaled into the 

 atmosphere by transpiration, while the remainder becomes acted 

 upon by the gases passing through the stomata. 



The atmosphere is composed principally of oxygen, nitrogen, 

 and carbon in the form of carbon-dioxyde or carbonic acid gas. 

 These are absorbed, the carbon and nitrogen are retained, and a 

 portion of the oxygen is exhaled, after having effected certain 

 chemical changes in the constitution of the fluid sap, and thus 

 fitted for its downward course, building up and solidifying the 

 various tissues, and perpetuating the life of the plant. These 

 changes in the condition of the sap are marked by an alteration 

 in the colour of the chlorophyll, which gradually loses its green 

 hue, and takes on the various shades of yellows, browns, and reds, 

 which clothe with such a charm the autumnal foliage of vegeta- 

 tion ; and the fibro-vascular bundles having served their purpose, 

 and being no longer needed for the transmission of fluids, become 

 constricted in the petiole, at the point of its insertion into the stem, 

 until the now withered leaf falls to the ground, to recommence, by 

 its decomposition, the wondrous cycle of vegetation life. 



Hitherto, our examination of withered leaves has been chiefly 



