by Mr. Alexander Montgomery, Jr., formerly of the Waban Rose 

 Conservatories, Natick, Mass., and now of the Montgomery Co., 

 Inc., Rose Growers, Hadley, Mass. 



THE PHYSIOLOGICAL EFFECT OF LIGHT. 



Practically ninety-five percent of the various substances which 

 make up the plant are derived from the atm,osphere through the 

 chemical action of light on the green coloring matter or chlorophyll 

 bodies, primarily located in the leaves. This process is tenned 

 carbon assimilation or photosynthesis, and consists in the taking 

 in of carbonic acid and the exhalation of ox}'gen. The carbonic 

 acid is broken down, and by combining with the water obtained 

 from the soil, forms the resultant product, starch. The spectinam 

 rays especially concerned in photosynthesis or carbon assim.ilation 

 are the orange and red, while those more particularly affecting 

 growth are the violet colors. Artificial light affects plants in pro- 

 portion to its intensity and the nature of its rays, and since it differs 

 from sunlight very materially, it cannot be substituted for sun- 

 light to good advantage unless some of the more objectionable 

 rays are screened out. 



Plants make the most growth in the night or in darkness, and 

 the least in the daytime. On the other hand, photosynthesis 

 takes place during the daytime or under the influence of light, 

 therefore these two important processes, — photosynthesis and 

 growth, alternate with one another. Although light has a marked 

 inhibitory effect on growth, it favors the development of m,echani- 

 cal or supportive tissue which is able to resist disease. Plants 

 grown in darkness are devoid of chlorophyll, and are consequently 

 whitish in color, with poorly developed leaves, elongated petioles, 

 internodes, etc. This shows the importance of light from the 

 physiological standpoint, and explains why the lack of it is often 

 responsible for many greenhouse troubles. Still, it must not be 

 overlooked that too m.uch light will under certain conditions in- 

 duce an outbreak of various troubles, so that shading the crop 

 often becomes necessary. 



There are a large number of plants,- — i.e., palms and others, 

 that do not require a great deal of light. There is reason to believe 

 that even the light comm.orf to our latitude is above the optimum 

 for som.e outdoor crops. The winter light in greenhouses is too 

 intense for some species; but carnations, roses, lettuce, cucumbers 

 and tomatoes invariably require m.ore light in the winter months 

 than they receive, consequently commercial growers should be 

 constantly on the alert for innovations in greenhouse construction 

 tending to give better light during this period. On the other hand, 

 light, as well as heat, which is associated with it, is often too in- 

 tense for certain greenhouse crops in sum,m.er, often developing 

 a tendency in certain plants to run to seed. Then too, the mois- 



