92 HISTORY OF BOTANY 



which, as you are aware, have pollinia or massed pollen 

 grains. After working out the morphology of the flower 

 and showing, with his usual acumen, how it is related 

 to the typical trimerous flower of the Monocotyledons, 

 he found that insects in their passage from flower to 

 flower carried the whole pollinium with them and that 

 several stigmas might be pollinated in succession by the 

 same pollinium. He next noticed the formation of the 

 pollen tubes from the pollen grains on the stigma and 

 traced them right down to the placenta, but there he lost 

 them. The extrusion of the pollen tube from the grain 

 had already been noticed by Amici in 1823 and by 

 Brongniart in 1826, but the significance of the pheno- 

 menon had not been appreciated. Brown found tubes 

 similar in appearance entering the micropyles, but he 

 could not convince himself that these were continuations 

 of the pollen tubes, though he conjectured that they 

 were at least derived from them. He accounts for the 

 protrusion of the pollen tube from the grain by assuming 

 that it received a stimulus from the stigma, and thinks 

 that the tube during its passage to the ovary is nourished 

 by something derived from the tissue of the style. This 

 nutritive relation between pollen tube and style was 

 worked out sixty years later by Reynolds Green. 



In the course of a corresponding enquiry into the 

 structure of the flower of Asclepias he carried the matter 

 a step farther by tracing the connection between the 

 pollen tubes and the tubes he had seen entering the 

 micropyles of the orchid ovules, and also recognised 

 granules inside these tubes, although he thought they 

 were concerned with nutrition and not with fertilisation. 

 It is in his paper on the Orchidaceae that he made the 

 great discovery that was destined in after years to lead 

 to the establishment of a new section of plant histology, 

 viz. Cytology. This observation of Brown's is of so 

 much interest that I will quote you the sentence in which 

 he announces his discovery : " In each cell of the epider- 



