186 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



hybrids of 38*5 per cent., compared with that of species crossed 

 with species. Why the male element should be affected more 

 than the female element in hybrids is not at all clear ; and 

 whether in time by constant crossing this would gradually be 

 eliminated I cannot say ; but I observe that while species crossed 

 with pollen of hybrids produce as we have seen 56*7 per cent, fertile 

 pods, yet 124 hybrids crossed with pollen of hybrids produced 

 seventy-four good pods, or 59*6 per cent., being an increase of 

 fertility of 2*9 per cent. This may possibly be due to the fact that 

 the hybrids crossed with hybrids were of necessity more nearly 

 related to one another than species crossed with hybrids, and it 

 is highly probable that by constant mixing hybrids may regain 

 the higher fertility usual among species and varieties. 



With regard to the lessened fertility of hybrids used as the 

 father compared with the greater fertility of hybrids used as the 

 mother, and its probable effect on the intercrossing of species 

 in a state of nature, is not for me to deal with now ; but it is of 

 great importance to the evolutionist as a possible factor in the 

 origin of species. All these experiments and observations prove 

 to us how important it is for Orchid breeders to keep accurate 

 and precise records of the crosses they make from time to time, 

 the details of which may prove to be of inestimable value to 

 science. 



THE DEPTH IN THE SOIL AT WHICH PLANTS OCCUR. 



By Prof. F. W. Oliver. 



[Kead November 0, 1897.] 



Many plants, especially such as are perpetuated by subterranean 

 rhizomes, tubers, bulbs, and the like, are characterised by the 

 particular depth in the soil at which these structures usually 

 occur. Thus the rhizomes of Aspidistra clatior creep at the 

 surface, those of Solomon's Seal at some little depth, whilst 

 those of Asparagus may be as remote from the surface as 10 to 

 16 inches. Now in cases in wljich the structure is normally 

 buried at a certain depth this is not attained all at once by the 

 seedling, but gradually, and often only after the lapse of several 

 years. The seed germinates at or near the surface, and during 



