PREFACE Vv 
In like manner the older botanists of today remember 
the incoming of the belief in the heteroecism of rusts, 
and how timorously the fact was accepted by teachers 
of good standing among botanists. And this hesitancy 
as to the acceptance of a new view was still more marked 
in regard to the nature of “lichens,’’ which by tradition 
formerly constituted a third group in the triumvirate of 
the lower plants, Algae, Fungi and Lichens—the “ thal- 
logens” of that day. Happily we have outlived this 
provincial timidity in regard to the startling conclusions 
of the German botanists, and in recent years have calmly 
accepted the substitution of a radically different system 
of the flowering plants for that which had generally pre- 
vailed for seventy-five years or more. Many of us still 
remember that the Gymnosperms used to be regarded 
as a division of the Dicotyledons, being sandwiched be- 
tween the Monocotyledons and the Angiospermous 
Dicotyledons. Now the Gymnosperms are regarded as 
belonging to a genetic line different from the Angio- 
sperms, although still associated with them as “seed 
plan 
It will be noticed that this book follows the usual 
German sequence of Morphology first, followed later by 
Physiology. ‘The experience of the authors leads them 
to think that it is better to give the student a good 
foundation in plant structure and then to have him study 
the plant in action. However, this does not require the 
teacher to defer all physiological topics until the com- 
pletion of Chapters I, I and III; indeed it has been our 
- practice to introduce such topics as soon as the student is 
prepared to master them. 
In the systematic chapters (II to XX) and especially 
in Chapter X XII the Plant Kingdom is divided into four- 
teen groups of primary rank, here called “‘phyla.”’ To 
