PREFACE V 



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 starthng 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 j^ears 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 

 plants." 



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, II 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 (VII to XX) and especially 

 in Chapter XXII the Plant Kingdom is divided into four- 

 teen groups of primary ranlv, here called ''phyla." To 



