THE AMERICAN BOTANIST. 77 



Trimorphic flowers, in which there are three lengths of sta- 

 mens and pistils are much rarer but several instances are 

 known. The most classic one, of course, is Ly thrum salicaria, 

 with which Darwin experimented but instances are known in 

 Oxalis, the pickerel weed {Pontederia) and the water willow 

 (Decodon verticillata) . To be fully fertile, each flower must 

 have pollen from the proper length of stamens in some other 

 flower, and the visiting bees soon get three bands of pollen 

 upon them, corresponding to the three different lengths of 

 stamens. In some cases, the pollen grains in the different 

 lengths of stamens, are of different sizes and appear to be 

 functional only upon pistils of the proper length. 



Change of Sex in Papaya.— It is pretty generally 

 known that some species of plants are dioecious, that is, some 

 individuals produce only staminate flowers, and others pro- 

 duce only pistillate ones. The tropical papaya (Carica papaya) 

 is one of these dioecious species, but it has recently been dis- 

 covered that under certain circumstances it can change its sex. 

 For instance, if one of the pistillate, and therefore unfruitful, 

 trees have its terminal bud removed it soon begins to produce 

 pistillate flowers and these later ripen good fruits. No reason 

 for such a change has yet been given but it will probably turn 

 out to be akin to that which c?<uses most plants to fruit when 

 their vegetative existence is threatened in any way. The con- 

 tinuance of the species is the most important thing in the plant 

 world, and the plants seem to make greater efforts to accom- 

 plish it than to perform any of their other functions. 



Plant Hairs and Moisture. — The petunia and various 

 other plants such as the tomato, tobacco and velvet-flower 

 (Salpiglossis sinuafa) belonging to the nightshade family 

 ( Solanacea-e) are well-known to resist drouth most success- 

 fully though they are not especially deep-rooted. Their ability 



