SUMMARY 73 



however, thanks to the evolution of the microscope, 

 matters were in a much more advanced state. The 

 classic monographs of Malpighi and Grew stand out like 

 lighthouses in the darkness, and there is nothing to replace 

 them until long after the dawn of the nineteenth century. 

 Although, as you are well aware, we are not, strictly 

 speaking, justified in talking of the development of the 

 idea of sexuality in plants until the discovery of the 

 actual gametes concerned in the process, still, accepting 

 the interpretation of the phenomena as then understood, 

 we may say that by the end of the eighteenth century 

 it had come to be accepted that the stamens corresponded 

 to male organs and the carpels to female organs, and 

 that the accessory parts of the flower were intimately 

 concerned in the process of pollination, or the application 

 of the pollen to the stigma, which had come to be recog- 

 nised as an essential preliminary to the formation of 

 the seed. What happened inside the ovary and what 

 part the pollen played in these happenings was as yet 

 a mystery. In the department of nutritive physiology 

 the hazy conceptions of Malpighi and of Grew, with 

 their weird ideas on the intermingling of juices and 

 subsequent fermentations, were beginning to give place 

 to conclusions based on definite experiments. Hales 

 stands out as the pioneer investigator on the new lines, 

 and to him we owe the foundations of our knowledge 

 of root pressure, transpiration, and the ascent of sap. 

 But undoubtedly the greatest advances in the theory 

 of plant nutrition were made after 1774 when Priestley 

 rediscovered oxygen gas. Ingen-Housz overshadows all 

 others in the closing years of the century as the dis- 

 coverer of the significance of the gaseous interchanges 

 taking place between the leaf and the atmosphere, and 

 his work, taking into account the condition of chemistry 

 in his day, must be regarded as one of the greatest con- 

 tributions to our knowledge of plant physiology that 

 has ever been made by any one man , 



