6 PREFACE. 



succeeding communication : — " It would 

 " appear from recent observations, that 

 " some plants are periodical in their isih^i- 

 " florescence; or, from some unknown 

 " cause disappear for several years at a 

 " time; for instance, the Orchis spectabilis 

 " was found by members of the late 

 " Botanical Society, 8 o/* 10 years ago; 

 " and, altho' the most diligent search has 

 " been made for it, for at least ^i;e years 

 " past, it has not been discovered until 

 " this season, when it is very abundant, 

 " and has been foun(l in various parts of 

 " the District, by different individuals. 

 " I have also observed, that the Arethusa 

 ** bulbosa, is very abundant some years, 

 *' and, during others, extremely rare. 

 " About five years since, Batschia canes- 

 " cew5, was found in great abundance, 

 ** near the Race-course, but has never 

 " been seen since. 



Washington^ October^ 1850. 



NOTE. ^ 



* The following is an extract from Mr. Collins' 

 letter on this subject — 



" PHiLAi)i5p»HrA, March 7th, 1822. 

 " Your plant I take to be Dracocephalum dentiadatum, 

 *' Ait. It agrees well with a specimen in my collection, so 

 *' marked, which many years since I brought from tliefai'm of 

 " Col. Graham, near where tlie Potomac marble is quarried." 

 *' but they might, perhaps, be considered as holding an inter- 

 " mediate station between that species and D. Virgmicum, if 



