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BowMAN: MECHANICAL TISSUE IN CERTAIN VINES 371 
vines but why there should be also a decrease in the mechanical 
tissues or at least an inefficiency in support for the stem can not 
be explained on this basis. The fact that reduced photosynthesis 
due to shade can bring about a decrease in constructive tissues 
might seem of some importance, but plants that have the habit 
develop characteristically in full light as well as shade so this 
argument is negligible. 
So it would seem that there must be some basis for argu- 
ment in support of the position taken before, viz., that in the 
light of de Vries’s classic experiment on Oenothera the mechan- 
ical tissue development has its origin in the unit characters 
of the plant. As these are partially dependent on nutriment 
in the ancestors it follows that there must have been con- 
siderable variation in the premutation stages of the vines studied. 
As de Vries says all mutations are not progressive, i. e., visibly 
differentiated, we may speculate and suppose the plant to have 
undergone a retrogressive mutation which produces a change, 
a decrease in the bast fibers, etc. These characters then of 
the normal type have become latent or suppressed and may be 
retained as internal units, e. g., in the Hedera and at a future time 
under proper conditions may become activated. When once the 
plant has become a “‘leaner’’ by deficiency in mechanical support 
the secondary result of diffuse light can produce its effects towards 
elongation. However, this is speculation and mere argument 
based on a posteriori grounds. As stated above, all mutations need 
not be advantageous, and the first stages in the vine habit may 
have been distinctly disadvantageous. It may be said that the 
primary variation in mechanical tissues is due to recombination of 
the characters of certain plants and that plants with vine habits 
arise as mutations due to the preceding causes. In conclusion a 
quotation from the Chicago Textbook (3, p. 656) may give the 
general idea of this line of evolution: ‘‘It is assumed, and probably 
correctly, that lianas have come from erect ancestors, and that 
their evolution was subsequent to that of trees. . . . Probably 
the first lianas were leaners, the twiners and tendril climbers 
developing later.” 
LITERATURE CITED 
1. Bary, A. de. Comparative anatomy of the vegetative organs of the 
phanerogams and ferns. English translation. Oxford. 1884. 
