CHAPTER II. 
ELEMENTARY MORPHOLOGY AND PHYSIOLOGY. 
Members of a Plant.—An ordinary plant, e.g., sunflower, 
regarded from a morphological point of view, is composed of 
parts which, though they present a great variety of shapes, can 
all be classified under four main categories—Hair-structure, Root, 
Stem, and Leaf. By repetition of these members a plant is built 
up, and they have even been regarded by some as individuals 
collectively forming a colony. All structures which come under 
one of these four headings, as say that of hair-structure or 
“‘ Trichome,” are homologues, and are said to be homologous, or 
to display homology. This signifies an agreement in relative 
position and manner of origin. It does not mean that they are 
necessarily similar in shape or perform the same office in the 
economy, although this may be and often is the case. All hair- 
structures agree in being superficial members, developed from a 
kind of membrane, the epidermis, with which plants are clothed. 
Anything which agrees with this definition is a hair-structure, 
whether it be a thread, simple or branched, a scale, or what not. 
And again, some hair-structures keep off unwelcome visitors, 
others (as in horse-chestnut leaf-buds) form a kind of glue to 
protect the young leaves from cold, while still others assist in 
processes which lead to the formation and scattering of the seed, 
and so on. 
Organs of a Plant.—If, on the other hand, a plant is regarded 
from a physiological standpoint, it is found to be made up of 
organs, 1.e., structures fitted to perform special kinds of work. 
Thus we have organs of nutrition, organs of respiration, con- 
cerned with breathing, &c., &c. Organs belonging to any one of 
these categories are analogues, and are said to be analogous, or 
to display analogy. Relative position and mode of development 
are here of no moment, the essential agreement being solely in 
the nature of the work performed. Now, although all hair- 
structures, roots, stems, and leaves are respectively analogous to 
a large extent, they are by no means entirely so. Hair-structures, 
for example, may (as mentioned above) perform very different 
