PARASITISM IN FLOWERING PLANTS. 
227 
from adaptive or “ homoplastic ” modifications fitting the plants- 
for the peculiar conditions of their life. The last-mentioned 
order has but very slight afi&nity with the two first, and indeed 
the gTOup hung but loosely together, and is now generally aban- 
doned ; all three orders are doubtless dicotyledonous with 
well-marked affinities to other orders of that class. 
Before we examine the peculiarities in question, it will be as 
well to recall to the mind of the reader the usual constitution 
of an ordinary phanerogam. Functionally, all its organs are 
either nutritive or reproductive ; that is, they are employed in 
processes which either contribute directly to the maintenance 
of the life of the individual to which they belong, or are 
directed towards the formation of new individuals in an embryo- 
state which shall reproduce the species. The former system of 
plant-organs includes the green leaves spread out in the sun- 
shine, the absorbent root-hairs buried in the soil or immersed 
in water — both food-obtaining organs, and both periodically 
renewed — and the whole system of axes, stem, branches and 
root, which carry these temporary structures, and are themselves 
of more solid and durable make, fitted to be not only supports,, 
but also channels through which pass in various directions, 
according to the necessities of the plant, the sap and other* 
juices. The structure of the widely separated leaves and root- 
hairs is very different, and adapted in either case to the sur- 
rounding conditions. The delicate walls of the latter readily 
allow the passage of fluid, and through them is constantly, 
streaming into the plant a current of water charged with car- 
bon-dioxide, and containing, copiously diluted, the varied mine- 
ral and nitrogenous constituents of the soil needed by the plant.. 
This fluid is then transmitted upwards from cell to cell, through 
the structure of the axial system, and is especially drawn to its 
extremities and to the leaves where growth is going on rapidly, 
and evaporation is great. The skin which covers these parts is,, 
on the leaves especially, provided with minute orifices (stomata), 
so that the atmospheric air freely passes into the loosely-built 
cell-structure below. The walls of these cells are thin ; the 
substance chiefly needed for the formation of plant-structures 
is carbon, and this is one constituent of carbon-dioxide, a gas 
which always exists in the atmosphere in small quantity as the 
result of combustion and animal respiration. This gas the 
leaf-cells extract from the air, it passes through the walls, and is 
reduced to its elements, almost the whole of the oxygen being 
returned to the air whilst the carbon is retained and assimilated.. 
At the same time are produced, by combination of the consti- 
tuents of the sap, the organic compounds, often very complex,, 
which are proper to the plant. These very remarkable processes 
occur chiefly in those parts which possess the characteristic 
