How Plants Resist Hostile Forces Around Them 273 



absorption, especially in very warm weather; and this might, if 

 too sudden, produce damaging distension and perhaps rupture 

 of the walls. But these plants are commonly covered by a thin 

 coating of jelly, which is known to greatly impede the rapidity 

 of water-passage, thus allowing enough time for an equalization 

 of pressures through the stem. This is very hkely the adaptational 

 significance of the jelly, or slime, of water plants generally. 



But the forces of the air, the earth and the waters are not the 

 only ones hostile to plants, for among their worst enemies are 

 other plants, and animals. As to plant enemies, the most 

 deadly are the parasites, the Bacteria, Molds, Mildews, Blights, 

 Rots, Rusts, Smuts, and other Fungi which often destroy their 

 host plants entirely. Yet few, if any, positive adaptations have 

 been found in plants protective against these parasites, although 

 some of the oils and resins occasionally found in leaves do appear 

 to afford a moderate protection against Fungi as well as against 

 animals. Practically all of these plants reproduce by spores 

 which are blown about by the wind; and when these fall upon 

 suitable plants they germinate into slender threads. These, 

 for the most part, have no power to penetrate the epidermis, 

 which is thus somew^ay protective against them; but they enter 

 the open stomata and thus reach the soft food-filled cells of the 

 leaf, which they proceed to devour. Thus the necessity for the 

 existence of stomata involves another danger besides that of 

 excessive transpiration, and in this case one against which plants 

 seem well-nigh helpless. However, plants differ immensely, — 

 not only different species but even different individuals of the 

 same species, — in their susceptibility to injury by parasitic 

 Fungi, and there is very good reason to believe that the differ- 

 ences have a chemical basis, some kinds possessing a chemical 

 constitution hostile to the growth of the parasite while others 

 do not. These differences offer a basis for the attempts now 

 being made in many experiment stations to combat plant 

 diseases by breeding unmune varieties, — the less susceptible 



