334 V. The Forestry Problem 



presence of which we seem helpless to apply preventive measures. All 

 trees subjected to infection do not in this case contract the disease, nor 

 do they in respect of larch disease, Nectria cinnabarina, Melamfsorella 

 caryophyllacearum and other leaf and stem parasites which could be 

 mentioned. Evidently, therefore, there are immune strains amongst 

 the host-plants of certain tree fungi, which suggests the idea of attempting 

 to raise resistant varieties by the application of the laws of breeding 

 which have given good results, with the promise of even better, in the 

 case of field and garden crops. But, here again, we are up against not 

 a fundamental but the practical difference between trees and other 

 crop-plants, namely, that in the latter the period of seed to seed — a 

 generation- — is often but a year, whereas in the former it is seldom less 

 than a quarter of a century. Then, again, in the case of at least one highly 

 important field and garden crop — the potato — having raised your 

 immune variety from seed you have the prospect of a commensurable 

 return through years of vegetative reproduction. Although such pro- 

 pagation is not unknown in forest practice {e.g. willows and poplars, 

 to some extent limes and elms) it is excluded in the case of most forest 

 trees, which either fail to strike as cuttings, or are hard and costly to 

 propagate as layers, and, at the best, turn out to be mis-shapen under- 

 sized individuals. But something can certainly be done in this direction 

 by the exercise of scrupulous care in selecting sound individuals as seed- 

 yielding plants, in the hope that immunity may be a Mendelian character, 

 and that the particular plant may be a homozygote, and that all or most 

 of its ovules may have been fertilised by the pollen of a similar individual. 

 And not only should the seed-collector keep predisposition to disease 

 in view, but other undesirable characters are also transmissible by 

 inheritance, such as feeble growth, twisted fibre, crooked and gnarled 

 stems, and excessive tendency to produce branches, with the consequent 

 result of knotty and inferior timber. These characters are latent in the 

 germ cell. When one sows a batch of tree seed in a nursery, one has the 

 experience of finding that, in say four years, a proportion of the plants 

 are much taller, and a proportion much dwarfer, than the others, and 

 one might be inclined to conclude that this state of things was the result 

 of the external conditions of growth on the particular individuals. No 

 doubt this cause can operate, but it is much loss influeutial than has been 

 supi)osed. In point of fact it lias been ])i'oved that the cell elements of 

 dwarf individuals are shorter than those of robust growth, and that the 

 quality of vigour is innate, and is probably a Mendelian character. The 

 question has an important bearing on economic forestry, inasmuch as 



