factor, such as light, and to what extent other factors, such as- 

 available moisture, may have contributed to it. In shady situ- 

 ations, in the case of one and the same species, not only may 

 leaves be found to be thinner, and often of a different colour,, 

 consistency, size, and shape, than they are in places exposed to 

 bright sunlight, but their anatomical structure may also be 

 fundamental i y changed, and we find sun-leaves with an epidermis 

 devoid of chlorophyll, provided with a thick cuticle, and with 

 well-developed palisade tissue, while shade-leaves often have 

 no palisade tissue and an epidermis with very thin walls and 

 provided with chlorophyll. Anatomical variations also may 

 be found in the petioles, stems and roots. Cases are known 

 of so-called dimorphic species which are able to maintain 

 themselves on high mountains and also in the lowlands, or 

 on dry land, as well as under the water, and variability is 

 thus seen to be a very useful character, inasmuch _ as vari- 

 able plants are thus enabled to adapt themselves to varying 

 conditions of existence and to widely extend their area of 

 distribution. Variations of the kind now under discussion 

 appear to us to be temporary and inconstant, and individuals 

 belonging to the alpine or aquatic form of a dimorphic 

 species, after existing for many generations on the moun- 

 tains or under water, are found to assume the characters 

 of the lowland or terrestrial form, respectively, when cul- 

 tivated in the plains or on dry land. At the same time cases 

 are known in which plants .under the influence of certain con- 

 ditions have undergone a slow but progressive change and 

 have thus acquired definite characteristics which have 

 become constant and hereditary. With regard to this point 

 the study of bacteria is of special value seeing that, in their 

 case, " experiments can be readily extended 



over a far greater number of generations than in the case of 

 flowering plants, for a bacterium which divides once an hour 

 passes through as many generations in ten days as an annual 

 plant does in 240 years."* By continued cultivation under 

 special conditions, it has been found possible to permanently 

 eliminate in certain species of bacteria the power of producing 

 poisons, pigments, and even the important character of the 

 power of spore production. Such changes do not occur sud- 

 denly, but only gradually become fixed and hereditary and 

 if, after short exposure to the special conditions, the bacteria 

 are returned to their original environment, the power of pro- 



* Physiiogy of Plants, by Dr W. Pfeffer, Volume 11. pages 192193. 



