322 



STUDIES IX LIFE AND SENSE. 



often named the " Telegraph Plant," ceases its movements when the 

 temperature is below 22 Cent. Desmodium appears, therefore, to 

 have overcome that dependence on light to which other plants are 

 subject, and exhibits a tendency to regard temperature as the ruling 

 condition of its life. 



There exists a striking analogy between the health and growth of 

 man or other animal and that of a plant, in respect of the influence 



exerted upon either by 

 light and darkness. As 

 the child grows stunted, 

 pale, and weak when bred 

 in the close, dark city 

 court or alley, and ap- 

 pears in striking contrast 

 to the healthy, ruddy- 

 complexioned country ur- 

 chin, so the plant, grown 

 in the darkness, contrasts 

 unfavourably with the 

 normal organism grown 

 in the daylight. Habit 

 and instinct in the ordi- 

 nary plant have apparently 

 moulded its normal con- 

 stitution in accordance 

 with the same laws which 



regulate the well-being of the animal. Experimentally treated, 

 the topic of the influence of light on plant-growth is best illustrated 

 by an experiment in which twelve seeds of Indian cress were placed 

 in three pots four seeds in each pot. The first pot was placed in 

 complete darkness, with the result that the seeds germinated only to an 

 extent compatible with the usage of the nourishing matter originally 

 inherent in their substance. Like a man living on capital, and deriving 

 no income from active work, these first seeds perished as soon as 

 that capital came to an end. In the absence of light, the chemistry 

 of the plants could not be exercised. Surrounded by soil and food, 

 they were unable in the absence of light to avail themselves of the 

 nutriment at hand. The second pot was, however, daily placed for 

 seven hours in daylight. At the end of three months, the plants had 

 gained in weight by five grammes. The third pot had continual 

 exposure to light, with an afternoon share of sunlight, and, in the 

 same space of time as that accorded to pot number two, the plants 

 had gained twenty grammes of dry weight. 



All parts of a plant, however, do not appear to require light as 

 a vital necessity, and this declaration may be extended to include 



FIG. 32. DESMODIUM, THE MOVING PLANT OF INDIA. 



