29 



evcnthiiit;' which has a l)caring upon the development of the 

 plant must he oaret'ull}- considered if a ]:)erfect type is to be 

 realized. 



Even when it is not possible to modify the heat, light and 

 moisture, as is the case out-of-doors, infection can be largely 

 eliminated by making use of certain cultural practices. 



Light affords a good illustration of the role a single factor 

 may play in the configuration of plants. The physiological effect 

 of light is to inhibit growth and to induce the formation of a firm 

 texture of the tissue. Lack of light stimulates growth, but plants 

 grown in darkness become etiolated and lack firmness of tissue. 

 They have small leaves and elongated petioles, and the epidermal 

 collenchymous and sclerenchymous tissues are undeveloped. 

 There are many instances of the absence of light being responsible 

 for serious troubles, while in other cases excess of light undoubt- 

 edly exerts a detrimental influence. The tonic influence of the 

 r.ordeaux mixture, brought about by favoring the formation of 

 chlorophyll and carbon assimilation in many plants would appear 

 to be due to the screening or lessening of the light intensity. Sun 

 scald, which occurs on various trees, is brought about by excessive 

 light, as in the case of apple trees, which, when defoliated by the 

 gypsy moth, usually die from the effects of sun scald. On the 

 other hand, shading often causes 'sun scald and winter killing by 

 retarding the ripening of the wood. 



In some cases too intense light or the conditions resulting from 

 it, apparently causes trouble, at least in greenhouses. In the 

 Northern latitudes many greenhouse crops do not obtain sufficient 

 light during the winter months, and when cloudiness prevails it 

 is with some difficulty that crops are matured without becoming 

 diseased. All expert greenhouse men mature their crops when 

 the weather conditions will permit, and not according to the 

 calendar; in other words, it requires a certain definite amount 

 of light, or so many light units, as it were, to mature a crop. 

 The light in May, for example, is equal in intensity and amount 

 to about twice that of corresponding periods of a day in Novem- 

 ber ; consequently, it requires about twice as much time to bring 

 a crop to the same degree of maturity in November as it would 

 in May. 



Lack of light is responsible for various mildews and leaf 

 spots, top-burn or tip-burn, wilts, etc. Many of these leaf spots 

 are seldom, if ever, found on plants to which sunlight has access. 

 The Sclerotinia diseases of lettuce, water cress and parsley are 

 likewise induced by crowding and shading, and light in such cases 

 will prevent infection by the formation of resistant tissue. It is 

 well known that absence of light causes the so-called "layering" 

 of wheat and "damping off" of seedlings and cuttings, and the 



