XIX.] THE CASE OF ZIZANIA AQUATICA. 329 



In this connection it may be interesting to recall 

 the attempts made late in last century to introduce 

 our wild rice, Zizania aguatica, into England. At first 

 the plants did not flourish in those cool, moist sum- 

 mers. Seeds from successive generations sowed them- 

 selves, and "in this manner the plants proceeded, 

 springing up every year from the seeds of the preced- 

 ing one, every year becoming visibly stronger and 

 larger, and rising from deeper parts of the pond, till 

 the last year, 1804, when several of the plants were 

 six feet in height, and the whole pond was in every 

 part covered with them as thick as wheat grows on a 

 well -managed field. Here we have an experiment 

 which proves that an annual plant, scarce able to en- 

 dure the un genial summer of England, has become, 

 in fourteen generations, as strong and as vigorous as 

 our indigenous plants are, and as perfect in all its 

 parts as in its native climate."* 



II. B. Acclimatization through variation in habit 

 in offspring is common and unequivocal. Variation 

 in habit is usually in the direction of lessening or 

 extending the period of growth. Many herbaceous 

 plants, when taken northward, start relatively earlier 

 in the spring and mature earlier in the fall than they 

 did originally. Others simply shorten their period of 

 growth without obvious change in the direction of early 

 vegetation. Of this latter class Indian corn is a good 

 example. "Thus the season required by maize varies 

 from six months in the elevated plains of Santa Fe, 

 in South America, to four months in the middle United 

 States and two and one -half months in the Eainy 



*Sir Joseph Banks, Trans. Lond. Hort. Soc. i. 22. 



