DIV. ii PHYSIOLOGY 291 



A similar influence of light on the polarity is shown by the egg-cells of Fucus and 

 Dictyota. 



Antithamnion cruciatum, one of the Florideae, forms decussately-arranged 

 branches when in diffused light ; on one-sided illumination the branches all stand 

 in one plane at right angles to the direction of the rays. Further examples of 

 dorsiventrality induced by one-sided illumination are afforded by the branches of 

 ' many Mosses, the thalli of most Liverworts, and the prothalli of Ferns ; these 

 structures in the absence of such illumination are sometimes radial and in other 

 cases bilaterally symmetrical. In fern prothalli and the thallus of Marchantia 

 the dorsal side is determined by the stronger illumination. In the case of the 

 prothalli, when the lower side is illuminated, the new growth is adapted to the 

 altered direction of the light and the former upper side becomes the lower ; in the 

 Marchantiaceous thallus, on the other hand, the dorsiventrality once induced 

 cannot be changed. The shoots of Ivy and other root-climbers in which the 

 climbing roots are produced on the shaded side may be cited as an example of 

 dorsiventrality induced by light in the higher plants. 



Comparison of an etiolated and a normal plant shows that influence of the 

 intensity of the light under which the plant has grown extends to the internal 

 structure. The tissues of the etiolated plant are less differentiated and thickened 

 cells are wanting. A less complete contrast than between light and darkness 

 may be effective. Shade-leaves ( 59 ) have a very different structure from the leaves 

 of the same species developed in full sunlight. They are thinner, their palisade 

 cells narrow below,leaving wide intercellular spaces between them, and form only 

 a single layer ; in sun-leaves the palisade cells are longer and form several layers. 



Alpine plants, the illumination of which differs in duration, intensity, and 

 composition from that in the plains, differ in their whole habit from lowland 

 plants. Their vegetative organs are contracted, while the flowers are large and 

 brightly coloured. Other factors than light are concerned in this change. 



3. Gravity. A plant can readily be removed from the light but 

 gravity is always acting upon it. It is only possible to change the 

 direction of its action. When the direction of the action of gravity 

 coincides with that of the main shoot and root of the plant no effect 

 is perceptible ; when it forms an angle with the line of these organs 

 curvatures are produced (see Geotropism), as in the case of illumina- 

 tion from one side. Apart from these curvatures an action of 

 gravity on the polarity of the plant is established ; this does not 

 amount, however, to inversion or to the transformation of the shoot 

 into a root. There is no case of the polarity of the undifferentiated 

 egg- cell being altered by gravity; this is always determined by 

 internal causes, though gravity may have a modifying influence. 



If twigs of Willow are cut and suspended in a moist chamber roots form near 

 to the lower end, while only the buds situated near the other end expand into 

 shoots (Fig. 261, 1). If the twig is hung in the inverted position it is the 

 corresponding buds at the end which is now lowest which still give rise to shoots, 

 while the strongest roots are produced near to the lower end which is now upper- 

 most (Fig. 261, 2). This experiment shows that internal causes mainly determine 

 the contrast of the two poles. Since, however, in the inverted position there is a 

 displacement downwards of root-formation and upwards of the unfolding of the 



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