896 ECOLOGY 



females. In dioecious species it has been claimed that large seeds are likely to 

 develop into pistillate plants, and small seeds into staminate plants. 



There may be noted some interesting cases of correlation, whose explanation is 

 not as yet forthcoming. Immediately after flowering it often is possible to distin- 

 guish at some distance pistillate from staminate mulberry trees by their much 

 smaller leaves, as though the constructive material in the former were utilized 

 chiefly in fruit development, and in the latter, in leaf development. Similarly, 

 in the box elder the leaves on flowering branches often are much smaller than 

 on vegetative branches. Later in the season, both in the mulberry and the box 

 elder, the leaves are equally large on pistillate, on staminate, and on vegetative 

 shoots. 



Among dioecious perennials (such as the box elder, poplars, and willows) the 

 same individual usually bears the same kind of gametophytes, regardless of external 

 conditions (even when transplanted as a whole or in the form of a cutting into a 

 very different habitat), so that two individuals which appear alike when not in flower 

 really are different, the one transmitting male attributes, and the other female 

 attributes. The gametophytes of Marchantia, for example, have been cultivated 

 vegetatively for thirty generations without undergoing any change of sex. There 

 are on record, however, some noteworthy cases of change of sex on the part of in- 

 dividual plants. Perhaps the best authenticated cases are those in which the sex 

 has been changed by wounding (traumatism). The primordia of pistillate inflores- 

 cences of maize have been subjected to torsion and thereby changed to staminate 

 inflorescences ; also staminate inflorescences have been changed to pistillate in- 

 florescences. By injuring the terminal bud of a staminate plant of Carica Papaya, 

 the plant has been stimulated to produce pistillate flowers which have matured into 

 fruits. Pulicaria dysenterica commonly has monoclinous flowers, but when the 

 subterranean organs are infested by Boris analis, an insect parasite, the species 

 becomes dioecious. The pistillate flowers of Lychnis dioica have stamen primordia 

 which rarely develop into mature stamens ; if these primordia are infested by a 

 smut ( Ustilago violacea), the stamens develop to a considerable size, though they 

 contain spores of the smut instead of pollen grains. A staminate grape used as a 

 stock for a monoclinous scion has been known to become monoclinous and to mature 

 fruit. In the strawberry, ordinary vegetative reproduction has been known to be 

 accompanied by sexual changes; in an imperfectly dioecious variety with mono- 

 clinous and pistillate individuals, the vegetative progeny of each kind of individual 

 has been known to develop into the other. A very remarkable change without 

 change of conditions has been reported for Aucuba japonica, in which a plant 

 that had been staminate for some years became monoecious and finally mono- 

 clinous. Similar changes have been reported for the lower plants, particularly 

 for Vaucheria, in which female branches have been transformed into bisexual 

 branches. 



Recent experimentation has resulted in a material change of view regarding the 

 significance of the influence of external conditions upon sexual development and 

 upon the change of sex, as noted in the preceding paragraphs. It is now generally 

 believed that in most plants the sex of an individual is not due to the external con- 

 ditions to which the individual itself may be subjected, but that sex is determined 

 much earlier than had been supposed. In the liverwort, Sphaerocarpus, sex is 



