606 Transactions of the Boyal Society of South Africa. 
a direct bearing on this subject, since the aim of Botany is to explain why 
plants are as they are. There is a danger, therefore, in restricting oneself to 
a single line of inquiry, neglecting the others. For instance, Warming (23) 
points out that the influence of the environment on species is complicated by 
hereditary tendencies, of which the causes are not yet understood. One 
result of this is that different species adapt themselves to the same environ- 
ment in widely different ways. To quote the example given by Warming : 
" While one species may adapt itself to a dry habitat by means of a dense 
coating of hairs, another may in the same circumstances produce not a single 
hair, but may elect to clothe itself with a sheet of wax, or to reduce its 
foliage and assume a succulent stem, or it may become ephemeral in its life- 
history." Warming is here obviously referring not to the environment as 
a whole, but to one main factor of the environment — in this case a " dry 
habitat.'' It may be doubted whether in Nature any two plants exist under 
absolutely identical environmental factors, though even if they did, they 
would not be identical in every respect, for it can hardly be supposed that 
the environments of any two species have remained identical throughout 
their evolution. If we had a full record of their past histories, we might be 
able to say why in reaction to dry conditions one species produces hairs, 
another wax, another reduces its foliage, and so on. It is hardly necessary 
to refer to " hereditary tendencies " until we have exhausted the possibilities. 
It is dangerous to lay stress on the reaction to one factor of the environment, 
neglecting the others, yet in most systems of classification of growth-forms 
this is what is done, and at the present state of our knowledge it is perhaps 
excusable. It is only to be expected, however, that species will be found 
reacting to that single factor when constant, in widely different ways, as in 
the examj)le quoted from Warming. 
On the other hand, different environmental factors may produce essentially 
the same growth-form. The effects of a dry atmosphere and " physiological 
drought," for example, may be the same, and when we take various combina- 
tions of external factors which differ among themselves, we often find more 
or less identical growth-forms resulting. The classification of growth-forms 
cannot, therefore, be based entirely on the adaptations resulting from definite 
and particular factors of the environment. 
A further point which has been noted by the writer as being particularly 
true with regard to the growth-forms of Natal plants, is that many species 
are so very variable that they belong to more than one growth-form. Many 
lianes {e.g. Scutia commersonii) often grow as stout trees, yet lianes and 
trees form two of the six different main classes of growth-forms according to 
Warming's system of classification. In the same way certain species come 
into any one of the four subdivisions of phanerophytes, according to 
Eaunkiaer's system (20, 22). 
Such difficulties and limitations are, of course, sufficiently clearly recog- 
