272 The Living Plant 



adaptation against damage by water has been claimed by those 

 who believe that transpiration is not simply an unavoidable 

 evil, but a process of value in itself. The presence of water on 

 leaves, derived from dew or the rain, must check transpiration, 

 partly by blocking the stomata, and partly by the creation of a 

 moist atmosphere around the leaves during its evaporation; and 

 any arrangements tending to prevent the wetting, or facilitate 

 the drying, of leaves would thus be protective. Such arrange- 

 ments do, apparently, exist in those waxy or other unwettable 

 coatings which enable leaves to shed water in small drops as one 

 can see readily in the Garden Nasturtium, the Pond Lilies, and 

 a great many others, some of which show a silvery film of air all 

 over the leaf when plunged into a vessel of water. The same 

 result is claimed to follow in a different way in those very many 

 leaves of tropical rainy regions which taper off to a long slender 

 tip ending in a very small point ; these tips collect, as it were, the 

 water-drops as they slip down the hanging leaf, and guide them to 

 the point whence their own weight makes them drip to the 

 ground, leaving the surface well drained, and ready the sooner 

 to begin transpiration. 



There is, however, one way in which water can damage plant- 

 structures directly, and it actually happens with grains of pollen 

 when these are touched by the rain. The functions and mode of 

 growth of these grains, which will be fully described in the follow- 

 ing chapter, is such that their walls are necessarily thin and their 

 contents osmotically attractive to water, whence it happens that 

 if they become touched thereby, they absorb it, swell up, and 

 burst, as can be seen very clearly when the water is added to 

 grains under the microscope. The pollen, therefore, needs pro- 

 tection from rain, whereto a good many adaptations have been 

 found, as can be considered, however, more appropriately along 

 with the Flower in the chapter devoted to that subject. It has 

 also been claimed that the surface-cells of the simple and soft- 

 bodied plants of fresh water are subject to a smiilar osmotic 



