204 Transactions. 



ortliostichies on either side approximate to one, and in L. scariosum two 

 dorsal ortliostichies bend over one to either side. In both species the 

 ventrally borne leaves, which catch no direct sunlight, become mere scales, 

 and in L. voluhile they become also very scattered and few in number. 



In sheltered, shady places plants of L. voluhile are found which show 

 the reversion foliage. Here no direct sunlight falls on the branches, the 

 light being difi'use, and the stimulus to a dorsiventral disposition of the 

 leaves is almost completely lacking. The equal all-round-the-stem stimulus 

 exerted by the diffuse light results in the tendency to a more all-round 

 development of the leaves ; or would it be more correct to say that the 

 inherited constitution of the plant has the opportunity to assert itself over 

 the acquired character ? Which is the correct way to express it depends, 

 of course, on whether or not the heterophyllous character which originally 

 came about as an epharmonic adaptation has altered the hereditable con- 

 stitution of the species. Some botanists, of course, are quite ready to 

 believe that such a thing is possible, and that this takes place in nature 

 more readily than is generally imagined, while others would hold that an 

 epharmonic adaptation must inevitably revert as soon as the controlling 

 stimulus is removed. In the young plants of L. voluhile and L. scariosum 

 the characteristic heterophylly appears while they are still erect in growth. 

 In L. scariosufn it appears almost from the very first, there being but a 

 few scattered scale leaves formed before the characteristic dimorphism is 

 in evidence, and there being practically no transition stages. In L. voluhile 

 the dimorphism of the leaves appears first when the plant has attained, 

 compared with L. scariosum, a considerable size (16, figs. 97, 98). It appears 

 first in some particular branchlet or other, and develops its characteristic 

 appearance in gradual stages, so that a plantlet possessing six to ten 

 branchlets will show probably every stage in the development. Since the 

 heterophylly appears in the plantlet before it has adopted the plagiotropic 

 habit, it would seem that, after all, this character has actually become fixed, 

 and that this has become so to a greater degree in L. scariosum than in 

 L. voluhile, for it appears there much earlier in the ontogeny. I have not 

 actually observed whether or not the young plants have their dorsiventral 

 branches turned at right angles to the direction of the light, but I should 

 judge that, if such were the case, heterophylly ought to begin in all the 

 branches at the same time. However, the plantlets grow generally amongst 

 thick moss and other vegetation where they get no direct sunlight at all. 



The Plagiotropic Habit. — The sporelings of L. voluhile, L.fastigiatum, and 

 L. scariosum maintain an erect growth for a much longer period than do 

 those of the species which belong to the Lnundata and Cernua sections. 

 The upright stems which arise from the protocormous rhizomes in L. cernuum. 

 L. ramulosum, and L. laterale, and the stems of the vegetatively produced 

 plantlets of L. Drmmnondii, almost immediately bend over and flatten 

 themselves in the plane of the ground, and a strong adventitious root emerges 

 at right angles from the stem and binds the latter to the ground. Whether 

 or not it is the earl}'' development of the first adventitious root which compels 

 the young plant to so soon adopt the plagiotropic habit, or whether rather 

 it is the strong plagiotropic habit which determines that even the first 

 adventitious root shall emerge at right angles from the stem at the point 

 at which it is given oft' from the vascular cylinder, and that it shall not 

 penetrate down the tissues of the cortex as it does in L. Selago, I am not 

 able to say. The fact remains, however, that the species which belong 

 to the lnundata and Cernua sections characteristically differ from those of 



