28 INTRODUCTION 



example of the fact that growth and development may be impossible in 

 spite of extremely favourable nutritive conditions. This is also the case 

 during the resting periods exhibited by higher plants, and frequently 

 germination can be induced in the unicellular spores of fungi and algae, 

 only after the lapse of a certain latent period, often amounting to weeks or 

 months. Reawakening may be made evident at once, or after a latent 

 period, by a renewal of the vital activity manifested as growth or in some 

 other form, while at the same time the fact is demonstrated that an 

 indirect or automatic alteration of the internal controlling factors may be 

 possible. Embryonic cells may be regarded as consisting of plastic 

 material, which within certain limits may be induced to grow and develop 

 in different modes and directions, according to the conditions under which 

 they exist. From this point of view, the facts already put forward teach 

 us how various tissue units and organs may be produced from originally 

 similar cells in consequence of the operation of internal causes and the 

 reciprocal influences of the developing parts. 



It is owing to the inductive influences to which their position renders 

 them subject, that particular cells, forming part of the primary meristem, or 

 derived from cambial division, elongate markedly and assume the peculiarities 

 which characterize the components of the vascular bundles. That these 

 cells do not in themselves possess an inherent power of always developing 

 in this manner is shown by the fact that if they are exposed by an intentional 

 injury they may develop differently and form callus or other tissue. More- 

 over, similar internal parenchymatous cells, when exposed peripherally by 

 the removal of the external tissues, may produce a new epidermis either 

 directly, or indirectly by a series of tangential divisions. The thallus of 

 Marchantia affords an instructive example of the inductive action of pre- 

 existent parts, for neither in the flattened apical cell, nor in the segments 

 immediately derived from it, is there any fixed dorsi-ventrality. The 

 inherent tendency to dorsi-ventrality is rendered definite and permanent by 

 exposure to light, and in the gemmae the side illuminated always becomes 

 the dorsal surface, even though it may happen in point of fact to be the 

 lower one. Since no reversal of dorsi-ventrality is possible when once 

 induced, it follows that the older parts must forcibly impress a dorsi- 

 ventrality similar to their own upon each successive new increment, what- 

 ever the existing external conditions may be. In the case of fern prothallia, 

 however, the influences radiating from the older parts are not sufficiently 

 powerful to prevent a change in the incidence of the illumination from 

 inducing a reversal of the original dorsi-ventrality on parts formed under 

 such new conditions. 



It is therefore the particular aggregate of causes brought to bear at 

 a given moment which determines whether from particular meristematic 

 cells, capable of developing in any direction, a leaf, a flower, or a leafy shoot 



