K.— BOTANY. 257 
not the right way to get a real understanding of the structure of a plant. 
At last year’s meeting of Section K the President, Professor Dixon, 
showed reason to believe that the sieve tubes of the phloem are in the 
cases which he considered quite inadequate for the purpose of carrying 
organic substances such as sugars from the leaves to the regions where 
they are used or stored, as, for instance, potato tubers. What, then, 
we may ask, is the ‘function’ of sieve tubes? It seems to me that 
we should not close our minds to the possibility that they may have 
no ‘function’ in this sense, that cells having the characters of what 
we call sieve tubes may quite conceivably be formed simply as the result 
of the processes going on in certain tracts of developing tissue, without 
subsequently playing any essential part in the economy of the plant. 
The analogy of the machine made by man, in which each part is 
constructed with a definite object, may be very misleading if we allow 
ourselves to forget that an organism is not constructed in that way at 
all, but is the outcome of blind, inevitable processes, and may produce 
parts which are useless or even harmful to it, provided that the whole 
is still able to ‘carry on’ and reproduce itself in its actual conditions 
of life. We should always approach structure through development, 
the mechanics, physics, and chemistry of growth and differentiation. 
It is only thus that we can ever hope to ‘explain’ structure in any real 
sense. It is only thus, I believe, that we can ever hope to get back 
to the real nature of the genes. 
The ‘ functions’ of the various organs and tissues—‘ biological ’ and 
‘ physiological’ functions in the old sense—will then appear in their 
proper places as those properties or activities which actually contribute 
to the growth, maintenance, and reproduction of the plant—for the 
plant must grow, maintain, and reproduce itself, or the race will die. 
The main essential activities are sufficiently obvious, and we can some- 
times say with confidence that if such and such a structure were absent 
or such and such a process did not take place, these essential activities 
would be fatally impaired. When a failure of this kind takes place 
owing to change of genotype or of environment we rarely see it, for it 
brings extinction in its train.?” For the most part we cannot know 
that apparently useful characters could not have been dispensed with, 
or that metabolic processes might not equally well have taken some 
other course so far as the success of the plant in the struggle for exist- 
ence is concerned, while in regard to a multitude of characters there is 
not only no proof but not the smallest reason to suppose that they 
have now, or ever did have, any ‘survival value’ at all. Like all 
structural features, they are simply products of the plant’s activity, 
though they react in turn to a greater or lesser degree on that activity. 
Differentiation and so-called division of labour are the inevitable result 
of increase in size, and of the ensuing different relations of parts of 
the body to one another and to the surrounding medium. Every type 
of plant, whether it differs from its parents or not, does and must 
20 In his ‘lethai factors’ the Mendelian geneticist has, however, succeed 
in discovering definite heritable entities which lead to such failure dere 
to death. ‘lhe real nature of these may be eventually ascertainable along the 
line of research indicated above. a 
© 2 
