PLANT HOEMONES 193 



hood. In the case of Convoluta, Roscoffensis, the 

 signal for the commencement of the later phases 

 of development owes its origin to the presence of the 

 green algal cells, without whose concurrence, prob- 

 ably by the production of a hormone, no kind of 

 artificial feeding has been found effective. 



"Mention may also be made of the substance ex- 

 tracted by rain from grass, which has been shown 

 by Pickering to be injurious to apple trees. They 

 should not, in fact, be surrounded by growing grass, 

 as is common in orchards." 



We know that a number of ethereal salts, or, as 

 the chemist calls them, "esters," act as accelerators, 

 in the sense of hastening the flowering of plants and 

 the ripening of fruits ; and we know further that the 

 essential oils present in plants are largely made up 

 of such "esters." As to whether these essential 

 oils are manufactured by glands similar to those 

 existing in the animal kingdom, and as to whether 

 there is any interrelationship between such glands, 

 cannot, at present, be answered. It is supposed 

 that these "esters," and substances other than 

 "esters" that act like accelerators, stimulate en- 

 zyme (ferment) action in the plant, particularly in 

 the later stages of its development. On this basis 

 the many color changes that take place during the 

 ripening period, the autumnal color of leaves, and 

 the dropping of the leaves from the stem of the 



