SECT. II PHYSIOLOGY 251 



the roots in their direction of growth react difterently to h'ght, gravity, 

 etc., than do shoots, it niight be supposed that the polarity of the 

 embryo was determined by similar external factors. This, however, 

 only holds for certain lower plants. Thus, for example, in the ger- 

 mination of the spores of Equisetum and of the eggs of the Fucaceae 

 and Didyota the direction of the first dividing wall, and therefore the 

 positions of the base and apex of the plant, is determined by the 

 direction of the light. Similarly an influence of gravity has been 

 shown on the first wall formed in the fertilised egg of Marsilca (^-). 

 As a general rule, however, internal and unknown factors determine 

 the polarity, and even when this is primarily eftected by external 

 factors it cannot later be altered by their influence. 



On the other hand, in some simply organised Siphoneae the internal polarity is 

 easily overcome by external influences. It is sometimes sufheient merely to reverse 

 the erect tliallus of Bryoijsis, to convert the fornier apical portion into a root-like 

 tube which penetrates the substratum and fastens itself to the grains of sand. It 

 has also been positively determined, although otherwise snch cases are unknown, 

 among the higher plants, that the growing points of the roots of Neottia and of 

 certain Ferns {Platy cerium Aspleniwn esculentum) may be converted through some 

 inherent tendency into the vegetative cone of a stem (cf. p. 47) C'^). 



The symmetry of the growing point generally depends on its 

 inherited internal nature. In some cases, however, it is possible by 

 a one-sided influence of light to convert a radial growing point into 

 a dorsiventral one (^^). Thus, for instance, Antithamnion criiciatum, 

 one of the Florideae, bears its successive branches decussately when 

 grown in diff"used light ; when illuminated from one side these 

 branches form in the plane at right angles to the rays of light. Other 

 examples of dorsiventrality induced by unilateral illumination are 

 aflbrded by the shoots of many Mosses, the thalli of most Liverworts, 

 and lastly the prothalli of Ferns ; these structures in the absence of the 

 one-sided illumination sometimes become radial and sometimes bilateral. 

 Thus, in Fern-prothalli and in Mardiantia the upper side is determined 

 by the stronger illumination. While, however, on inverting prothalli 

 the new growth adapts itself to the altered relations to light and the 

 former upper side becomes the lower surface, in the case of the thallus 

 of Marchantia the dorsiventrality when once established cannot be 

 chanired. 



In Adventitious Formations, on the contrary, the influence of 

 external forces is often very evident ('^^), as, for example, in the forma 

 tion of climbing-roots, which in the Ivy and other root-climbers are 

 developed only on the shaded side of the stem. In the Alga Caulerpa 

 the new leaf-like organs arise only on the illuminated side of the parent 

 organ. It is, on the other hand, the force of gravity which excites 

 the formation of roots on the lower side of underground I'hizomcs, and 

 determines that new twigs develop, for the most part, from the upper 

 side of the obliquely growing branches of trees. Contact stimuili, on 



