MINUTE STRUCTURE OF PLANT HYBRIDS. 207 



other in affinity, particularly since we know that such are frequently less fertile than the 

 pure product of either parents, or are entirely sterile. The objection will afterwards be 

 considered, but here I may premise that, as a rule, whether the parents are remotely or 

 closely related their evenly blended peculiarities appear, if comparison is at all possible. 



To the above general conclusion, however, we must make an important exception. 

 In not a few cases, which will afterwards be cited, a separation or prepotency of the 

 sexual molecules of each parent seems clearly to be indicated. 



(e) Relative Stability of Parent Forms. — Some species show both in the wild state and 

 under cultivation a greater degree of stability, or want of variation tendencies, than do 

 others. This is probably to be explained by an average structure having been slowly but 

 steadily evolved through crossing and recrossing of an aggregate of like individuals with 

 survival of those best fitted for a set of environmental conditions that remained constant 

 through long periods of time. These, therefore, even when removed to rather dis- 

 advantageous surroundings do not readily exhibit change. As examples, I may name 

 Erica Tetralix, E. cinerea, and Philesia buxifolia. 



One finds that the opposite is equally true of not a few species. Thus, if a series of 

 individuals of Geum rivale or Dianthus barbatus (cultivated) be compared microscopically, 

 considerable variation is traceable. 



But even species which are considered to vary little, if compared from wide areas, 

 may present unexpected changes. An interesting illustration is furnished by a plant just 

 cited as one of the most invariable, viz., Erica Tetralix. I have shown elsewhere* that 

 this species resolves itself into four sub-species, three of which are found in Connemara, 

 and these, so far as they have been experimented on, remain true under cultivation. It 

 is necessary, therefore, in the selection of a hybrid to know the exact type of each parent, 

 if not the actual parent, and to examine such alongside the hybrid offspring. 



II. Comparison of Hybrid Structure with that of the Parents. 

 (a) Philageria Veitchii, x . 

 I have chosen this hybrid — in many respects the most remarkable yet produced — 

 as the first type for detailed examination, so that anyone who has not the histological 

 sympathy necessary to the task of wading through the details of other types may 

 acquaint himself to some degree with the relation of a hybrid to its parents. My 

 choice has been made chiefly because the walls of its elements are so evidently in- 

 termediate throughout between those of its parents. It should be stated, how- 

 ever, that it does not readily furnish us with illustrations of protoplasmic and allied 

 modifications which less striking hybrids present. It was raised in the nurseries of 

 Messrs Veitch at Exeter, by the crossing of Lapageria rosea with pollen of Philesia 

 buxifolia. A very good description, with figure, was given by Dr M. T. Masters in 

 the Gardeners' Chronicle^ who successfully epitomised its history in its name. For 



* Trans. Bot. Soc. Edin., vol. xix., 1891. t Gard. Chron., p. 358, 1872. 



