How Substances are Transported and Removed 211 



that the water-drops always appear. Guttation, therefore, is a 

 kind of a safety-device for the plant even if transpiration is not. 

 Furthermore, it happens at times that roots keep their vitality 

 long after the stems have died, and continue to force up water 

 which can find an outlet only through rifts that it makes in the 

 withering stems. Besides, in cold weather all stems tend of 

 course to contract, thus squeezing from such rifts any over- 

 abundant water they may happen to contain. When water from 

 either of these sources is forced out in cold weather, it freezes in 

 lines, which soon become flat plates as more and more issues 

 from the stem, pushing the already formed ice before it ; and this 

 is the origin of the ice crystals or shells, often of great beauty and 

 commonly mistaken for " frost," which are seen on the stems of 

 some plants in the early part of the winter.* 



If I seem to have dwelt over-long on this matter of water- 

 removal from the plant, I claim in explanation that the process, 

 because of the profundity of its effect upon plant-structure and 

 habit, is worth all the space I have taken; and the later chapter 

 on Protection will help to support this conclusion. But now we 

 are ready to proceed to the topics remaining, of which the re- 

 moval or excretion of substances other than water comes naturally 

 next. These excretions belong to four different classes. First, of 

 course, are the gases, for oxygen is an excretion in photosynthesis, 

 and carbon dioxide in respiration. But the subject is simple, for 

 they pass off by diffusion, either through stomata and lenticels 

 of leaves and stems, or in solution through the wet epidermis of 



* A conspicuous case occurs in Helianthemum canadense, commonly called Frost- 

 weed, which is described in Gray's Manual of Botany thus: "Late in autumn crystals 

 of ice shoot from the cracked bark at the base of this and the next species, whence 

 the popular name." Another, and even more striking, example is the Dittany 

 (Cunila Mariana, or origanoides) , in which the ice-forming habit has thus been de- 

 scribed: "Our Cunila has attached to the stem a shell-work of ice, of a pearly white- 

 ness, beautifully striated, sometimes, like a series of shells one in another at others 

 curved round on either side of them like an open, polished, bivalve; then, in others, 

 again, curled over in every variety of form, like the petals of a tulip." (J. Stauffer, 

 quoted in the Botanical Gazette, XIX, 1894, 326.) 



