254 SECTIONAL ADDRESSES. 
and reaction’ conception, with its postulate of a series of mysterious 
mechanisms, supposed to have been built up by natural selection, and 
apparently inaccessible to further analysis. That this conception was 
a necessary stage in the development of plant physiology we need not 
deny; but some physiologists, like some of their morphological col- 
leagues, seem to have rather mistaken a transitory stage of development 
for an ultimate condition of research. Within the last few years we 
have begun to get developmental physiological studies of all kinds, and 
some of these are at last beginning to give us some insight into the 
formative processes which result in the differentiated structures of the 
plant body. A number of years ago Goebel, in his ‘ Experimentelle 
Morphologie,’ sketched the connexion between various characteristic 
external forms of plants and definite factors of the environment. In 
1916 one of my predecessors in this chair, Professor Lang, clearly 
outlined the ideal of ‘ causal morphology,’ and indicated lines on which 
he thought such investigations should proceed. It is, I think, quite 
possible to claim that ‘causal morphology’ in the widest sense is 
morphology proper; to say, with Professor D’Arcy Thompson, that 
since the problems of form are in the first instance mathematical prob- 
lems, and the problems of growth are essentially physical problems, 
‘the morphologist is ipso facto a student of physical science.’’7 More 
recently, again, Professor Priestley and his collaborators have attacked 
with considerable initial success the question of the actual sequence 
of events leading to the differentiation of various tissues, more par- 
ticularly endodermis, cork and cuticle, and have perhaps opened the 
way to a causal ontogenetic understanding of the whole of the tissue 
systems of the higher plant.** 
It certainly seems a far cry from a causal knowledge of these onto- 
genetic processes, common to whole families or large groups of plants, 
to an understanding of the way in which the genes which determine 
the difference of phenotype between one species and another, or one pure 
line and another, bring about the development of the corresponding 
phenotype. Superficially at least the kind of character whose origin 
in the ontogeny Priestley and his fellow-workers have been investigating 
seems to differ in nature from the kind of character which commonly 
separates species and varieties. The one is built into the constitution, 
and helps to determine the economy not only of one species but of a 
wide range of related species or of great groups of plants; the other, so 
far as the vital economy of the plant is concerned, often seems to be of 
no importance at all. To use a metaphor which is perhaps just per- 
missible, the difference is like the difference between the plumbing of 
a house and the decoration of its facade, or between the lay-out and 
17 )’Arcy Thompson, On Growth and Form, 1917, p. 8. 
18 T am aware that there are some physiologists who think that this line of 
attack is overbold, that our existing knowledge of biochemistry and physiology 
does not justify a direct attempt to grapple with such problems. I can only 
say that I am not in agreement with this criticism. The results reached seem 
to me already to justify the methods employed, though, of course, it may well 
be that some of Professor Priestley’s first conclusions will have to be revised 
in the light of future knowledge. 
