SOME PROTECTIVE DEVICES OF PLANTS. 21 



nected directly with the trunk or branch is a bundle of vessels and 

 fibres which spread a:part and ramify in the leaf so as to give the 

 blades their various shapes. These ribs or veins, — they serve both 

 for support and conduction, — have spread over and between them a 

 thin mass of delicate structure called the mcsophyll, while enclosing 

 the whole is a thin transparent skin known as the epidermis. The 

 mesophyll is made up of those life units known as cells, each con- 

 taining living protoplasm and a grgen coloring matter, and bounded 

 by a thin soft transparent cell wall. This aggregate of cells can best 

 be compared to a large number of thin rubber sacs fitted and fas- 

 tened together to form a layer several times the thickness of one sac. 

 If an elastic membrane be imagined as enclosing the whole aggrega- 

 tion, and every sac as distended by air, it will be seen that the form 

 and firmness of the whole body will depend very largely on the dis- 

 tension of the units. The distension or turgidity of the leaf cells 

 depends on a sufficient supply of water. The water enters the plant 

 tmder the influence of a force known as diffusion, which is as in- 

 flexible as the force of gravitation. Hence the leaf cells, being 

 unable to control the inflow of water, must adapt themselves to per- 

 mit the escape of excess, and to conserve any quantity less than the 

 optimum. This is done in many ways. The most notable method is 

 by temporarily assuming a position of protection when exposed to 

 too great a loss of water. We call this change of position wilting, 

 usually implying merely a collapse due to loss of turgidity. It is, 

 however, much more than this, as we find that the position taken is 

 always one which defends the leaf from further injurious loss. T^o 

 permit the escape of superfluous moisture, the nearly waterproof 

 epidermis is pierced by innumerable minute openings on the lower 

 or shaded side of the leaf. These stomata are controlled by guard 

 cells which collapse and close the orifice when the loss of water is 

 too great. The change of position known as wilting assists this pro- 

 tective process by either rolling the leaf with the stomata inside as 

 is the custom among grasses, or closing the lower surfaces together 

 as in clovers, peas, and beans, or by the more common method of 

 drooping, so that the sun's rays strike the leaf obliquely, and are re- 

 flected instead of being permitted to penetrate. The aspect of an 

 assemblage of plants during the excessive heat of a summer day 

 should therefore not be considered as indicating a condition of hope- 

 less collapse. Rather it represents well designed attitudes of defence, 

 which the individuals will quickly relinquish when the danger is past. 

 A different class of protective devices are the modifications in 



