6 TOPOGRAPHY OF CHLOROPHYLL APPARATUS IN DESERT PLANTS. 



SCOPE AND PURPOSE. 



Field-studies on the transpiration of desert plants when in a leafless con- 

 dition, either as a result of the usual seasonal changes, the advent of 

 drought, or normally without leaves, lead to the discovery that with deli- 

 cate apparatus* the evolution of watery vapor can be demonstrated when 

 it might be least expected and in surprisingly large amounts. Some of the 

 plants thus studied were Cereus giganteus , Echinocactuswislizeni, Fom/nicria 

 sfi/cndcns, I\<cbcrlinia spinosa, Opuntia ro'sicolor, Parkinsonia micro/i/ivl/a, 

 and others.! This work early suggested an examination into the extent of 

 chlorophyll and the character of the chlorophyll-bearing tissues in the con- 

 stant parts of the plants. As opportunity offered the work was carried on. 

 and it demonstrated so much of interest that a summary was presented before 

 the Botanical Society of America, New Orleans, December, 1905. 



Any satisfactory study of the chlorophyll relations of the desert plants 

 must take into account the peculiar light conditions to which they are ever 

 exposed. In the present study no attempt has been made to do this, in part 

 because of the complexity of the subject, in part because of the lack of sat- 

 isfactory instruments for making light measurements. It therefore has 

 been limited to an observation of the chlorophyll apparatus as it exists, 

 without reference to correlations other than the obvious biological ones 

 which cropped up everywhere throughout the entire course of the work. 



METHODS AND MATERIAL. 



In such a research as the present one living material at hand is a prime 

 necessity, for the reasons, which are very obvious, that chlorophyll can be 

 most satisfactorily identified in living material, and, furthermore, a large 

 quantity of material is a necessity from which to select what is representa- 

 tive as well as by which to know the range in variation of the structures to 

 be studied. 



While unusual conditions have been taken into account, this paper aims to 

 present primarily the usual and normal condition of the chlorophyll appa- 

 ratus. In every instance conclusions were drawn from the study of only 

 normal and healthy plants, and with but one exception {Parkinsonia ac/i- 

 Icata} the plants were studied in their proper habitats. 



The developmental method of study was employed. That is to say, vig- 

 orous branches or stems were selected and sections were made at measured 

 intervals from the tip. Whenever necessary, comparative observations, in 

 addition, were made on mature structures, so that in each instance the story 

 might be as complete as possible. The presence of chlorophyll in a stem 



*Cannon, W. A.: A new method of measuring the transpiration of plants in place. 

 Bull. Torr. Bot. Club, 1905, 32 : 515. 



tCannon, W. A.: On the transpiration of Fouquierias plendcns, Bull. Torr. Bot. 

 Club, 1905, 32:397; and Biological relations of certain Cacti, The American Naturalist, 

 1906, 40 : 27. 



