EtJNNmG OUT OF VAEIETIES. 



25 



rate work of Doctor Nilsson, whose work at Svalof, Sweden, is so 

 well known through the writings of De Vries. Some of the work on 

 vegetatively propagated species will be given in dealing with the 

 effect of selection on this class of plants. 



RUNNING OUT OF VARIETIES. 



It is quite generally believed that there is a tendency for vegeta- 

 tively propagated varieties to ''run out." This subject has received 

 much discussion but very little careful investigation. There is no 

 question that in many species such varieties do lose vigor and become 

 practically worthless after a few years. Carnation growers all agree 

 that varieties of these plants are short lived. Varieties of carna- 

 tions seldom retain their vigor a dozen years. It is generally be- 

 lieved that varieties of potatoes retain full vigor only for a few decades. 

 With no selection this is undoubtedly true. We do not know just 

 what effect careful selection to maintain yield might have on the 

 length of life of a variety of potatoes. It is certain, however, that 

 the old Peachblow potato, so popular half a century ago, has been 

 maintained in full vigor by selection to the present time. Mr. E. H. 

 Grubb, of Colorado, one of the leading potato growers of that State, 

 is now growing this variety and finds it an excellent yielder. It is 

 probable, however, that potato varieties do tend to run out. The 

 same may be said of apples, but, as in the case of potatoes, definite 

 investigations on this point are lacking. 



On the other hand, there are species that have completely lost the 

 power of producing seed, as, for example, the banana. These have 

 been propagated vegetatively for ages without loss of vigor. But 

 we can not say that the same variety persists indefinitely, because 

 the facts are wanting. It is probable that among vegetatively 

 propagated races we may find every gradation between races which 

 run out very quickly and those which remain vigorous indefinitely. 



A good many species of plants produce seeds parthenogenetically," 

 as the dandelion, certain species of Hieraceum, etc. These are 

 among our most vigorous weeds. How long they have propagated 

 asexually we do not know. They may, however, be cited as in- 

 stances of plants which apparently retain their vigor through long 

 periods without recourse to sexual propagation. 



What has been said of races propagated vegetatively applies 

 equally to those which are habitually self-fertilized, with perhaps 

 the difference that in most such species there may occasionally occur 

 cases of cross-fertilization. It is quite generally assumed that 

 self-fertilized races tend to run out. There is some evidence that 



That is, without fertilization. In these plants the reduction division is omitted 

 in those mother cells which develop into seeds. 

 165 



