70 The Plant World. 



"Tuckerman states that his longest specimens were a little more 

 than a foot in length, and that the widest perforated expansions 

 exceeded 20 mm. Just what dimensions this plant may attain I 

 am not prepared to state, but I agree with Dr. Peirce that it is 

 probably the largest of our North American lichens. Specimens, 

 four or five feet long are common on the trees about Stanford 

 University, their widest perforated expansions reaching two 

 feet or more. Specimens in my herbarium vary from those 

 of the most delicate lace-like structure to those having broad, 

 unperforated expansions 25 to 40 mm. in width, dimensions 

 much greater than the largest perforated specimens seen by 

 Tuckerman. Usnea longissima Ach. attains a length of eight or 

 nine feet on the redwoods of the Santa Cruz Mountains, but the 

 thallus is so slender that its real size is much less than that of 

 Ramalina recticulata. 



The plant is evidently such a rapid grower that last Sep- 

 tember a series of measurements were made, at the suggestion 



of Dr. G. J. Peirce, in order to obtain some definite results 



The winter rains did not begin for nearly two months after the 

 measurements were made, and ceased two or three weeks before 

 the lichens were re-measured, so that most of the growth was con- 

 fined to a period of about five months. During the late winter 

 and early spring growth must have been both rapid and contin- 

 uous, since for nearly two months it rained almost daily, the 

 temperature also being favorable for growth." Mr. Herre also 

 records similar results obtained for " Parmelia caperata (I,.) Ach." 

 of his paper, corrected in ink in my copy by Mr. Herre to "Par- 

 melia soredica Nyl." 



That undisturbed growth, and a prolonged growing season 

 are undoubtedly important factors in luxuriance of growth is 

 most probable, but that "moisture" (^rainfall, fog, and humid- 

 ity) also is one of the main factors is not based on a mere guess, 

 nor on the general opinion of lichenologists, but on a year's 

 study by the author of growing plants, the conclusive results of 

 which investigation will be found in this magazine (11 :54. 1908. 

 PI. 6) The statement is reprinted here. "During the past year 

 I have measured the growth monthly of two examples of this 

 species {Parmelia caperata), with the following results, though 

 no doubt substratum, the character of its surface, the exposure, 

 and the climate have much to do with the rapidity of growth. 



