8 
A. G. Tansley 
to the first, which bring about growth in length, we find that as this 
progresses the apical meristem gives rise to a structure of gradually 
increasing elaboration, till the highest complexity of primary 
structure attained by the species is reached. 
Thus while the whole ontogeny of the typical animal is a 
gradual metamorphosis of the egg into an embryo, and of the 
embryo into an adult organism, the greater part of the ontogeny of 
a vascular plant is a process of addition in space, a building up bit 
by bit of an increasingly complex structure upon the original simple 
structure. In this way there is retained a record of the ontogenetic 
development in the first formed parts of the axis, a record which in 
the animal body is largely lost as development progresses. 
The comparatively complex structure eventually attained by 
shoot-axes of vascular plants which reach any considerable size, 
cannot of course be adequately supplied with water and dissolved 
mineral food from the soil through the slender channel furnished 
by the first formed primary axis. Either the primary root must 
branch and form new absorptive organs, itself increasing in size and 
complexity by secondary thickening so as to form a sufficient 
conducting channel between the newly formed absorptive organs 
(its branches) and the shoot-axis, or new roots must be continually 
produced on the shoot-axis itself, so that the more complex parts 
of the shoot-axis with its branches are directly supplied from the 
soil and are not dependent on the first formed parts of the plant at 
all. 
The former method has been adopted by the typically dominant 
modern plants, the dicotyledonous and coniferous trees; and we 
have palaeontological evidence that it was adopted also very largely 
by the dominant arboreal Palaeozoic and Mesozoic plants. This 
method is clearly the condition essential to the successful establish¬ 
ment of the erect arboreal habit in which the shoot-axis rises a 
considerable distance above the surface of the soil. 
The latter method on the other hand is characteristic of the 
great group of Ferns, with which these lectures are concerned, as 
well as of nearly all the other existing Pteridophytes, and of many 
herbaceous Angiosperms as well. The erect habit, and a considerable 
stature, have, it is true, been attained by certain plants, both Ferns 
and Monocotyledons, which have no secondary thickening. Without 
entering into the different methods by which the Tree-ferns and the 
Palms have been able to establish the arboreal habit, it may fairly 
be said that they are of the nature of short cuts to success, and 
