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! has 
pointed out how Vacecnium 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 
bas been seen in many species of Selaginella 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 Psclotum 
complanatum, with its bilateral symmetry already mentioned. Of the 
Lycopodiales the early fossil types were characteristically radial in 
1 Organography, Feng. ed., vol. i., p. 94. 
ganograp Es 
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