8 THE STUDY OF PLANTS IX 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 formulae. 



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 phanerogamic (i.e. 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 



