166 BOTANY part i 



with the development of roots ; while the systems themselves also 

 exhibit widely extended division of labour. 



The Ontogeny of the Form and Structure of Plants 



Just as the phylogenetic development of the vegetable kingdom 

 has proceeded from the simple to the complex, every plant in its 

 ontogeny passes through a corresponding development. The study of 

 the ontogenetic development is termed embryology, and the commence- 

 ment of the ontogenetic development is spoken of as germination. 



However a plant may arise, whether from an asexually produced 

 spore or from a fertilised egg, its first inception is always as a single 

 cell. In unicellular, spherical, or rod-shaped organisms, such as 

 Gloeocapsa j^olydermatica (Fig. 1) or Bacteria (Fig. 4), the whole course 

 of development is concluded with the cell division which gives birth 

 to two new independent organisms (Fig. 1). If the cell divisions be 

 continuous and parallel, and the newly developed cells remain in 

 contact, CELL FILAISIENTS (Fig. 4 a*) will be formed ; if the division 

 walls have different inclinations, and are at the same time all in the 

 same plane, cell surfaces are produced ; and if the walls are formed 

 in three dimensions of space, cell masses are the result. Such an 

 organism will attain but a low degree of development if all its cells 

 have a like value. It resembles then a unicellular organism save for 

 the fact that the cells produced by division remain united. With the 

 distinction into BASE and apex a plant manifests a higher degree of 

 differentiation. A vegetative or growing point is then developed, 

 usually at the apex (Fig. 5). This retains its embryonic nature, 

 while the parts produced from it attain a mature condition, a 

 distinguishing feature of the ontogeny of plants as compared with 

 animals (p. 4). The behaviour of growing points has been already 

 described (p. 101), but the processes occurring in the ontogenetic 

 development of the plant require further consideration. A qualitative 

 difference in the embiyonic tissue takes place at a short distance 

 below the growing point. There the embryonic tissue loses its 

 meristematic character, and becomes transformed into the differentiated 

 body of the plant. As a general rule, in plants with an epidermis, 

 primary cortex, and central cylinder, the epidermis is developed from 

 the dermatogen, the primary cortex from the periblem, and the central 

 cylinder from the plerome. This differentiation of the tissues does 

 not take place in all cases ; an already determined separation of the 

 several tissues must not be assumed to extend to the embryonic 

 tissue, the peculiar cell arrangement of which is due rather to physical 

 causes. The vascular bundles must pass through the periblem in 

 order to reach the leaves. The periblem is therefore capable of 

 producing, not oidy the primary cortex, but also the vascular 

 bundles and accompanying tissues of the central cylinder. The terms 



