i8 PLANT LIFE. 



aquatic plants in their attempts to gain a footing on dry 

 land, favoured by the method for preventing dessication 

 described above, this idea was replaced by a second, which 

 proved so successful that it has never been superseded, 

 and is at the present day the universal method adopted by 

 all forms of terrestrial vegetation. This successful effort 

 depended on division of labour, the one idea more than all 

 others that has enabled life to reach its present phase of 

 development, and consisted in the external cells of those 

 portions of plants growing in the air becoming converted 

 into a waterproof epidermis or skin, the various structural 

 peculiarities of which will be explained at a later stage. 

 The protective organs of plants may be conveniently referred 

 to under two headings. 



(a) Protection against climate. Under this heading may 

 be mentioned hairs, movements (as the closing up of leaves 

 and flowers), colour, etc. 



(b) Protection against living enemies. Stings, spines, bitter 

 tastes, colours, &c. It is important to remember that the 

 above divisions are not sharply marked in nature ; for 

 example, colour may be protective against both climate and 

 living enemies; the same is true of various scents and 

 secretions. Flowers may close during a rainy day to 

 prevent their pollen being washed away, or the closing may 

 take place at certain periods to prevent the entrance of 

 insects not adapted to effect fertiUzation of the young seeds. 

 Many contrivances evolved obviously for protective purposes 

 by plants are rendered more or less useless by the cunning 

 of living enemies ; for example, the common wayside weed, 

 Glechoma hederacea, or Ground Ivy, has a corolla prolonged 

 behind into a tube which secretes honey for the purpose of 

 attracting insects in connection with cross fertilization. 



