INTRODUCTION, " xxv 
trary definition of teratology or to trace the limits 
between variation and malformation, it may suffice to say 
that vegetable teratology comprises the history of the 
irregularities of growth and development in plants, and 
of the causes producing them. These irregularities 
differ from variations mainly in their wider deviation 
from the customary structure, in their more frequent 
and more obvious dependence on external causes rather 
than on inherent tendency, in their more sudden ap- 
pearance, and lastly in their smaller lability to be 
transmitted by inheritance. 
What may be termed normal morphology includes 
the study of the form, arrangement, size and other 
characteristic attributes of the several parts of plants, 
their internal structure, and the precise relation one 
form bears to another. In order the more thoroughly 
to investigate these matters it is necessary to consider 
the mode of growth, and specially the plan of evolution 
or development of each organ. ‘This is the more 
needful owing to the common origin of things ulti- 
mately very different one from the other, and to 
the presence of organs which, in the adult state, 
are identical or nearly so in aspect, but which never- 
theless are very unlike in the early stages of their 
existence.’ Following Goethe, these changes in the 
course of development -are sometimes called metamor- 
phoses. In this way Agardh’ admits three kinds of 
metamorphosis, which he characterises as: 1st. Suc- 
cessive metamorphoses, or those changes in the course 
of evolution which each individual organ undergoes in 
' Wolff was the first to call attention to the great importance of the 
study of development. He was followed by Turpin, Mirbel, Schleiden, 
Payer, and others, and its value is now fully recognised by botanists. 
* Agardh, “ Theoria Syst. Plant.,” p. xxiii. 
