14 THE STUDY OF PLANTS IN ANCIENT AND MODERN TIMES. 
course, and that accordingly every species is constructed on a plan fixed within 
general limits and exhibiting variation in externals only. These, it is true, are often 
more conspicuous at first sight than the direction and disposition of the parts which 
are really fundamental, and secure the stability of the entire structure. But in 
order to ascertain the plan of construction it was found necessary to go back to the 
very first visible appearance of each organ; to determine how the original rudi- 
ments of the embryo and the beginnings of roots, stems, leaves, and parts of the 
flower are formed, and to see what rudiments succeed in opening out, branching and 
dividing, and what remain behind to perish and be displaced by organs growing 
vigorously in close proximity to them. 
These researches into the course of development of the separate parts of flower- 
ing plants, and to a still greater extent the observations of the development of 
cryptogams or spore-plants (rendered possible by improvements in the construction 
of microscopes), led naturally to a study of the history of the elementary structures 
of which all plants are composed. Previously three kinds of elementary organs had 
been supposed to exist, utricles, vessels, and fibres. The observations of Brown and 
Mohl (1830-1840) resulted, however, in the identification of the cell as the common 
starting-point of all these elementary organs. This led to the further discoveries 
that protoplasm is the formative and living part of a cell, and that each cell is 
differentiated into a protoplasmic cell-body and a cell-membrane. It followed 
that the envelope of the protoplasmic body, the cell-membrane, which had hitherto 
been considered the primary formation, was in reality a product of the protoplasm 
enveloped by it, and this discovery resulted in a complete revolution in the con- 
ception of cells generally. Further investigation led to the conclusion that the 
various modes of growth and multiplication depend on definite laws. That even 
in the mode of juxtaposition of daughter-cells arising in reproduction, a certain plan 
of construction may be distinguished in each species which must stand ultimately 
in some causal relation to the structural system of the whole plant. The progress 
achieved along these lines in the course of a few decades has been extraordinarily 
great, no doubt due to the peculiar fascination which the study of the life-histories 
and transformations of living organisms and the observation of mysterious processes 
invisible to the naked eye have had for the mind of the inquirer. 
In that group of plants which includes the forms classed together by the earlier 
botanists under the name of Cryptogamia an altogether new world was revealed. 
An undreamed-of variety was discovered to exist in the processes of propagation 
and rejuvenescence of these forms of plants by means of single cells or spores. 
Objects which, having regard to their external form, had been assigned to widely 
different groups, were found to be connected with one another as stages in the 
development of one and the same species; and one result of these discoveries was 
the establishment in this division of the vegetable kingdom of an entirely new 
system of classification based on life-histories. The systematic arrangement of 
Flowering-plants or Phanerogams also underwent essential alteration. The Linnean 
system, founded on the numerical relations between the different parts of the flower, 
