IMPROPER FOOD AND POISONS. 473 



untoward influence of cold, by careful selection of seed for a 

 series of years, has been successfully answered by cultivators in 

 some northern countries of Europe. 1 



1236. Among the protective adaptations of seedlings to cold 

 is that described by De Vries, 2 who has noted that in certain 

 instances there is a marked retraction of the caulicle into the 

 ground upon the approach of a lower temperature. The with- 

 drawal is due to the contraction of the cellular tissue composing 

 the root. 



1237. Effects of too intense light upon the plant. All other 

 conditions being natural, living plants containing chlorophyll can 

 perform their functions normally when placed in the brightest 

 sunlight. 8 Even when the rays of light are moderately concen- 

 trated upon the foliage by a large convex lens there is no seri- 

 ous disturbance of function. But when, as in Pringsheim's 

 experiments (see 824), the sunlight is rendered very intense, 

 assimilation is arrested and destruction of the protoplasm soon 

 ensues. 



1238. Effects of improper food upon the plant. It has been 

 shown (Chapters VIII. and X.) that certain substances are in- 

 dispensable to the healthful growth of plants ; and it has further 

 been pointed out that most of these substances may be offered to 

 the plant in excess with no marked results. It should now be 

 noted that a few of these substances, notably nitrogen com- 

 pounds, applied in excess may induce a more luxuriant growth 

 than is desirable to the cultivator. Penhallow 4 and others have 

 pointed out that certain maladies of plants are largely dependent 

 upon malnutrition. In such maladies fungi are frequent con- 

 comitants, in many cases invading plants already enfeebled by 

 improper or insufficient food ; in others, obviously causing by 

 their presence and activity the diseased conditions. 



1239. Effects of poisons upon the plant. Noxious Gases. The 

 most hurtful of these, considered from a practical point of view, 

 come as products of the combustion of inferior sorts of coal, 



1 Schiibeler (see note on page 465). 



For an account of the formation of ice in plants, and the different degrees 

 of temperature at which it takes place, consult Miiller : Landwirthschaftl. 

 Jahrhiicher, ix., 1880. 



2 Botanische Zeirung, 1879, p. 649. Haberlandt has also examined the 

 same mechanism to some extent. 



8 It is a familiar fact that many plants thrive best in deeply shaded glens. 

 Success in the cultivation of such plants is attained only by regarding their 

 natural condition. 



* Houghton Farm Experiment Department, series 3, no. iii. 



