8 THE STUDY OF PLANTS IN ANCIENT AND MODERN TIMES. 
the point of view of their life-history, and endeavoured to trace them back to their 
origin. Tracing the development, from one stage to another, of all the different 
species, of the multitudinous forms of leaves and flowers, and of the various kinds 
of cells and tissues, the student of this school has to detect identity in multiplicity, 
to show that the connection between forms which have arisen from one another is 
in accordance with fixed laws, and to express those laws in definite formule. 
The attention of botanists was in the first place directed to the wonderful series 
of changes in the form of the leaf which occur in all phanerogamie (7.c. flowering) 
plants as the delicate seedling gradually turns into a flowering shoot. At the circum- 
ference of the stem which constitutes the axis of the plant, foliar structures are 
produced at successive intervals. All these structures are essentially the same; but 
they exhibit a continuous modification of their shape, arrangement, size, and colour, 
according to their relative altitudes upon the stem. To discover the causes of this 
structural variation was an attractive problem, and very diverse theories were 
suggested for its solution. The earliest explanation, which was given by the Italian 
botanist Cesalpino in 1583, is founded rather on superficial analogies and remote 
resemblances existing between tissues than on careful observation. According to 
this theory the stem is composed of a central medulla highly endowed with vitality, 
and surrounded by concentric layers of tissue, those namely of the wood, the bast, 
and the cortex. Each of the foliar structures put forth from the axis is supposed to 
originate in one of the above-named tissues, the idea being that the green foliage- 
leaf and calyx grew out from the cortical layer, the corolla from the bast, the 
stamens from the wood, and the carpels from the medulla. It was believed, also, 
that the outer envelope of a fruit arose from the rind of the fruit-stalk, the seed- 
coats from the wood, and the central part of the seed from the medulla. 
Early in the eighteenth century there came to be connected with this theory the 
doctrine of so-called “ prolepsis,” which was founded on more accurate comparative 
observations. It was thought that the medulla of the stem breaks through the rind 
at particular spots to form at each a bud, which subsequently grows out into a side 
branch. Owing to this lateral pressure of the medulla the ascending nutrient sap 
becomes arrested beneath the rudimentary bud, and, in consequence, the cortex 
develops under the bud into a foliage-leaf. In the bud the different parts of the 
future annual shoot are already shadowed forth in stages one above the other; and 
each is produced always by the one beneath it. As soon as vegetative activity is 
resumed after the expiration of the winter rest, the bud sprouts. If only that part 
of it develops which constitutes the first year’s rudiment, a shoot furnished with 
foliage-leaves is produced. But the embryonic structures belonging to succeeding 
years, which are concealed in the bud, may also be stimulated to development; and 
when this happens, these premature products do not appear as foliage-leaves, but 
in more or less altered forms as bracts, sepals, petals, stamens, and carpels. If no 
such anticipatory activity has been excited, the rudiment which in the previous 
case would have developed into a bract does not appear till the following year, and 
then as a foliage-leaf; whilst that which would have formed a calyx in the first 
