52 A TEXTBOOK OF BOTANY [Ch. Ill, 7 



accompaniment of photosynthesis, while secondarily it per- 

 forms the functions of lifting the minerals into the leaves, 

 and at times of neutralizing excessive solar action upon 

 exposed surfaces. 



Connected indirectly with transpiration is guttation, 

 frequent in young herbaceous plants. It occurs at those 

 times when roots are supplying water forcibly and abun- 

 dantly, but transpiration is checked. The surplus water is 

 then exuded through water pores (which are modified 

 stomata), at the ends of the veins, where it collects in glisten- 

 ing drops, commonly mistaken for dew. The drops can be 

 made to appear by experiment, and are often seen in garden 

 plants on cool mornings after hot nights, or even on warm 

 humid dull days; while often in cool evenings after hot 

 afternoons the water drops run down and wet the foliage, as 

 familiar in Cannas. In some measure related to guttation 

 is the formation of shell-like ice on the stems of certain 

 herbaceous " frost plants" in early winter; for the water 

 freezes as it is forced from cracks in the dying stems. 



7. The Adjustments of Green Tissues to Light 



Food formation is the first function of plants, and takes 

 place only in chlorophyllous tissues under action of light. 

 Accordingly it is natural that plants should exhibit special 

 adjustments of their green tissues to the sun. 



Most prominent of such adjustments is the existence of the 

 leaf itself ; for the leaf is simply a thin sheet of chlorenchyma 

 provided with accessory veins, air spaces, and epidermis. In 

 any typical foliage leaf, as observation indicates and micro- 

 scopical measurement confirms, the chlorenchyma is re- 

 markably uniform in thickness throughout all parts of the 

 blade, in which respect it differs greatly from the veins. 

 Furthermore, the chlorenchyma of all foliage leaves, no 

 matter whether small, as in Mosses, or great, as in Palms, is 

 not far from the same thickness. Exact measurements of the 

 cross sections of many common leaves show that in different 



