IN PTERIDOPHYTES 209 



that they are more highly susceptible of modification of symmetry than 

 is the strobilus ; and so they have naturally been the more frequent 

 subject of enquiry and of experiment, the observations chiefly relating 

 to the post-embryonic shoot. The dorsiventral construction of the 

 vegetative shoot is very common in creeping and climbing plants in 

 the most different circles of affinity. It also appears in the lateral 

 shoots of plants of which the primary shoot is radial. Dorsiventrality 

 may make itself apparent either in unequal development of the leaves 

 (anisophylly), or in difference of their position ; or it may also affect the 

 form of the stem itself. It may be found in one plant that outer 

 influences may directly bring about the dorsiventrality, while in others 

 it may exist from the beginning, and be hereditary. Goebel 1 has 

 pointed out how Vacdnium Myrtillus shows in its lateral shoots a 

 transitional state between these two cases ; for in the lateral buds of this 

 plant there is an influence exercised, probably by light, which leads 

 to a distichous arrangement of the leaves; but it does not take place 

 in all buds alike : in some the effect is only a secondary one, acting 

 upon the leaves which originate in a spiral succession : in others the 

 effect is primary, acting upon the vegetative point itself, on which 

 accordingly the leaves arise. The existence of such gradations of effect, 

 between dorsiventrality which is the result of immediate impress of outer 

 influences and that which is an hereditary condition, is important as 

 suggesting how the more fixed dorsiventrality may have come into 

 existence. The comparison of such cases, and of the vegetative system 

 at large in a number of allied plants, leads to the conviction that in the 

 vegetative shoot as well as in the strobilus the radial was the primary- 

 type, and the dorsiventral the derivative. The causes are probably the 

 same in both cases. It is, however, essential to note that the vegetative 

 region is more liable to be influenced by them than the fertile; for it 

 has been seen in many species of Sclagiiiclla and of Lycopodium that 

 the vegetative shoot is dorsiventral, while the strobilus is radially con- 

 structed. The same is the case with many of the Coniferae. Such 

 examples indicate that the strobilus is more conservative of form than 

 the vegetative shoot. It is true the converse may be found in some 

 of the higher Flowering Plants ; for instance, in the Labiatae the vegeta- 

 tive shoot is commonly radial, while the flowers are dorsiventral. But 

 this condition of the flower is probably one of the relatively late 

 specialisation. 



Examining more particularly the vegetative region of the Pteridophytes, 

 the radial type of shoot is found with high constancy in the Equisetales, 

 both fossil and modern. Also in the ancient Sphenophyllales and the 

 modern Psilotaceae : the only exception in the latter being Fsi/otitm 

 rnmplanatiim, with its bilateral symmetry already mentioned. Of the 

 Lycopodiales the early fossil types were characteristically radial in 



1 Organography, Eng. ed., vol. i., p. 94. 

 O 



