REPRODUCTION 375 



the Bermuda grass (Cynodori) and quack grass (Agropyron), have 

 extremely efficient asexual methods of reproduction. 



Some plants which normally do not reproduce asexually, may 

 be so propagated by means of "cuttings," which are merely 

 pieces of stems that are removed and stimulated to produce ad- 

 ventitious roots, thus establishing connection with the soil. Many 

 shrubs, trees, and herbaceous plants are propagated commercially 

 in this way, including the grape, banana, rose, and begonia; 

 sweet potatoes are propagated entirely in this fashion. Some 

 workers have insisted that vegetative reproduction continued for 

 long periods of time results in deterioration and have cited the 

 potato, which frequently does deteriorate with age, as proof of 

 their contention; but it has recently been shown that this deteriora- 

 tion in the potato is produced by filterable virus diseases and does 

 not occur in strains lacking the virus. Furthermore, the observa- 

 tions of plants known to have been propagated asexually for even 

 thousands of years indicates that there is no inherent defect in the 

 method as such. The yam (Dioscorea) has been grown vegetatively 

 in China for over 2,000 years, and there is no evidence that it is 

 deteriorating. The sweet potato has lost the power of sexual 

 reproduction in temperate climates and has been cultivated for 

 many generations without apparently losing its vigor. Similarly 

 the fig, date, and grape have been grown vegetatively for centuries 

 without showing any signs of becoming decadent; and the saffron 

 crocus (C. sativus), which is sterile, has been grown from the asex- 

 ual, lateral corms since the Middle Minoan period (1900-1800 B.C.), 

 showing that it is possible to reproduce a plant asexually without 

 any visible deterioration for nearly 4,000 years. 



Grafting is a form of asexual reproduction by cuttings where 

 the cutting or scion is caused to grow in the stem or stock of another 

 plant instead of in the ground. Although plants as unrelated as 

 the cabbage and tomato have been successfully grafted together 

 (Daniel), for the practical success of this operation the two plants 

 which are grafted together must be of congenial relationship, 

 i. e., they must not be too unrelated taxonomically. One can 

 graft apples and pears together as well as tomatoes and potatoes, 

 but one cannot graft successfully apples and cherries, which, it is 

 interesting to note, give negative results also with the precipitin 

 test (Chap. XIV). Furthermore, plants must be taken which have 

 lateral meristem (cambium), since it is this region which grows 



