480 PHYSIOLOGY OF GROWTH. 



placed at 18 C. under a bell-glass standing in water. If, e.g., we 

 experiment with pollen grains of Narcissus Tazetta (7 per cent, 

 cane-sugar solution), we find that many grains have germinated 

 even after six hours. But only the pollen grains near the edge of 

 the cover-glass have developed tubes, the rest owing to want of 

 Oxygen have not germinated. The tubes almost all grow towards 

 the middle of the drop ; they exhibit distinct negative aerotropism. 

 It is however to be observed that there are many kinds of pollen 

 whose tubes are in no way aerotropically irritable (Orobus vernus, 

 etc.). 



According to my experience the growing pollen tubes of species 

 of Lathyrus are excellent objects for the determination of nega- 

 tively aerotropic behaviour. The pollen grains are sown in sugar- 

 gelatine, containing 15 per cent, of cane-sugar. 



Many pollen tubes (Narcissus Tazetta, N". poeticus, Fritillaria 

 imperialis, Yincetoxicum officinale, etc.) also exhibit very vigorous 

 chemotropic irritability. 



We distribute the pollen grains in the sugar-gelatine drop, and 

 lay in the middle of this a fragment of freshly cut stigma or style 

 tissue or ovules of the corresponding plants. The grains germinate, 

 and the tubes developing in the neighbourhood of the fragment of 

 stigma, etc., being positively chemotropic, turn towards them. If 

 we kill the fragments of stigma, etc., by dipping into hot water 

 before laying them on the drops of gelatine, the pollen tubes 

 behave exactly in the same way. This proves that the direction 

 of growth observed in the tubes in the case before us cannot be 

 due to their aerotropic irritability, since the masses of tissue when 

 dead do not, as when living they of course do, produce Carbon 

 dioxide, which might influence the distribution of Oxygen tension 

 in the preparation. 



Frequently in investigating chemotropism in pollen tubes, it is 

 well to put them into a drop of gelatine containing only small 

 quantities of sugar (e.g. '2-4 per cent.), then lay on the fragment 

 of stigma, etc., and put the preparation in a moist place. 



Cane-sugar, which occurs in the cells of the gynasceum, and is 

 secreted by its cells, is one of the most effective stimulants for 

 the tubes. 



We inject leaves of a Tradescantia (e.g. T. discolor) under the 

 air-pump with cane-sugar solution of certain strength. The now 

 translucent leaves are rapidly rinsed with water, and dried 

 externally with blotting-paper. We now powder pollen on the 



