PREFACE. 
The present paper is the fourth of a series by Dr. J. N. Rose, 
entitled ‘*Studies of Mexican and Central American Plants,” the 
earlier numbers of which were published in previous issues of the 
Contributions from the United States National Herbarium. Since 
the preceding number of the Studies was published, Doctor Rose has 
made a fourth journey to Mexico, and a part of the information and 
material thus acquired by him has been utilized in the present paper. 
Throughout the progress of the work the scientific value of this field 
experience has been manifest, and the wisdom of the policy of giving 
the critical student opportunities of this kind has been confirmed. 
Freperick V. CoviLie, 
Curator of the United States National Herbarium. 
Ill 
