338 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



THE EVOLUTION OF PLANTS, AND THE DIRECTIVITY OF 

 LIFE, AS SHOWN BY VEGETATIVE STRUCTURES. 



By Rev. Professor G. Henslow, M.A., F.L.S., V.M.H. 



[Read June 17, 1913 ; Sir F. W.Moore, F.L.S., in the Chair.] 



Introduction. — My last lecture * was on " The Origin of Life — 

 Undiscoverable." We can only deal with life so far as we know it. 

 I endeavoured to show that all theories of the origin of life were 

 inadequate ; and one reason was because they left out of considera- 

 tion the fact that life is something over and above all kinds of physical 

 force, and that, in the process of making vegetable and animal struc- 

 tures, life guides or directs inanimate forces so that they compel the 

 similarly inanimate matter to build up cells, tissues, and organs in a 

 purposeful manner. 



We group the various organs of plants under two heads, the 

 Vegetative and Reproductive (proper) . The former embrace the roots, 

 stems and branches, leaves and stipules. The latter are the flowers 

 and fruits. The floral organs are the bracts, sepals (calyx), petals 

 (corolla), stamens, and carpels (pistil). Each and all of these 

 organs are variable, i.e. can change their forms provided the necessary 

 external conditions of life are present. 



As long as a plant lives generation after generation under precisely 

 the same conditions, no change is likely to occur ; but when seeds 

 get transported to some markedly different locality, then, as they 

 germinate, they begin (if it be necessary) to change their structures 

 in response to the changed environment. The change may be slight — 

 as in a greater or lesser degree of hairiness — or profound, especially in 

 the internal structures, as in changing from a land to a submerged 

 life, and vice versa (as has occurred in the water-crowfoot). The 

 " response " may bring about no " benefit " to the plant, as in the 

 case of drought, which prevents branches and leaves being fully 

 formed, so that they get reduced to spines, a common feature of 

 tropical " thorn forests," savannahs, deserts, heaths, &c. On the 

 other hand, the changes generally involve uses, without which the 

 plant could not live ; as in developing a cuticle and breathing 

 stomata, on what had been submerged leaves without either 

 structure. Hence we may say generally that " purposeful adapta- 

 tions " are the result of changed conditions in the environment, such 

 being ultimately the effect of the " Directivity of Life." 



I now propose giving illustrations of such changes in each of the 

 several organs of plants. 



Roots. — Roots have three principal functions, viz. to supply 

 water and dissolved salts ; they may act as reservoirs of water and 



* See p. 39. 



