291 



Conditions are similar with our fruit trees, most of which rest a year, 

 that is to say, bear one year a smaller crop and then the next a larger one. 

 After a heavy crop the trees are usually so exhausted that they need one 

 year in order to store up sufficient nutritive substances for the next crop. 

 Hoffman^ mentions further that many trees (the horse chestnut and the 

 Scotch Pine) exhibit a normal alternation of sexes, since they bear staminate 

 flowers one year and perfect ones the following. The increase of carpels 

 in the giant poppy (Papaver somniferum forma polycarpica monstrosa) 

 occurs only in the most vigorous plants. During his travels Karsten^ found 

 that the palms growing in swamps and damp woods, as a rule, bear perfect 

 blossoms, but become polygamous again from a lack of nutrition. The 

 genera growing on dry cliffs or arid plains have ordinarily but not naturally 

 separated sexes, and these bear staminate and pistillate flowers on separate 

 branches. At the beginning of the dry season the fruit ripens, requiring a 

 great deal of nutritive material, and then only staminate flowers develop; 

 while after the dormant period, at the beginning of the rainy season, pis- 

 tillate blossoms are formed in great abundance. 



Cugini^ found in starved plants of maize, which he obtained by heavy 

 seeding, that various individuals bore only staminate flowers. De Vries* 

 was also able to demonstrate the inheritance of sterility in the case of maize. 

 He took seeds from plants in which the pistillate inflorescences were entirely 

 wanting or extremely weak and obtained in the first year 12 per cent, of such 

 imperfect plants. The sowing of the following year yielded 19 per cent, of 

 sterile plants. 



A case described by Miiller-Thurgau^ shows that aside from nitrogen 

 hunger sterility can often be due to a lack of moisture alone. He found 

 the stigmas on fruit trees so dry that the pollen grains could not germinate. 

 In comparative test experiments with pears, trees which had been abundantly 

 watered during the time of blossoming exhibited an evident increase in yield. 

 Not only did numerous blossoms on the unwatered trees fall, shortly after 

 the time of blossoming was past, but even the young fruits, when about the 

 size of cherries, fell in strikingly large numbers. On trees standing in dry 

 places, usually one fruit remained to each umbel, while in the case of water- 

 ed trees, on an average, three developed. 



But sterility occurs even with good pollen and with stigmatic conditions 

 favorable for germination. Waite'' in his experiments on pear blight kept 

 insect visitors away from the flowers and found that the fruit set to a very 

 small extent. Further investigations convinced him that certain varieties 

 of pears and apples cannot he fertilized at all by their own pollen (nor by 

 that from other individuals of the same variety), but that the pollen from an- 



1 Bot. Zeit. 1882, p. 508. 



2 Linnaea, 1857, p. 259. 



3 Cugini, Intorno ad un anomalia della Zea Mays. cit. Bot. Centralbl. 1880, p. 1130. 



4 de Vries, H., Steriele Mais als erfelijk Ras. Bot. Jarbook II, p. 109. 



5 III. Jahresber. d. Versuchsstat. Wadensweil. Zurich, 1894, p. 56. 



6 Cit. Galloway, B. T., Bemerkenswerftes Auftreten einiger Plflanzenkrankheiten 

 in Amerika. Zeitschr. f. Pflanzenkrankh. 1894, p. 172. 



