LIGHTS Light constitutes one of the most important external factors affecting vegetation, and plays a prmuinent rile in modifying the configuration of plants. Photosynthesis, or the assimilation of car bon, is one of the most fundamental processes in the vegetable kingdom, and is dependent on light. The activity of this process increases proportion ally to light intensity.
Except in the polar regions, plants are exposed to the influence of light during only half their life period ; the other half is spent in darkness. So far as is known, plants do not assimilate carbon during bright moonlight nights, although sensitive appli ances for determining light intensity are capable of registering the comparatively feeble illumina tion of even bright nights, which would tend to show that the minimum amount of light necessary for photosynthesis is comparatively strong. Pho tosynthesis takes place under the influence of elec tric and artificial lights, as has long been known, but the activity of the process depends on the in tensity and the spectrum of the particular kind of light.
In glasshouse and other intensive cultures, it is important to know whether artificial lights of any kind can be used economically to supplement sun light and thereby produce an earlier or better crop. It is also important to know what effect artificial lights have on plants in exhibition halls. This subject has been the theme of considerable experimenting, but little very practical agricul tural result has yet been secured. In the winter, particularly in cloudy climates, artificial light may very likely come into prominence in the growing of some kinds of crops. The following account gives a brief survey of what has been accom plished.
Electric arc light.
Many experiments have been made relating to the influence of electric light on vegetation, more particularly with the stronger lamps, such as the arc light. The spectrum of the ordinary electric arc light is that of carbon, with a slight addition, in some cases, of the spectra of certaih gases. It is especially rich in the rays of high intensity, lying in the ultra-violet or actinic part of the spectrum, beyond the range of vision. It is well known that electric light possesses more of the ultra-violet rays, with probably less of the orange rays, than sunlight; therefore it would not be ex pected that electric light would possess the same value to plants as sunlight, even if the intensity of each were the same, since the rays which are the most valuable for photosynthesis are those located in the yellow and orange hands of the spectrum.
On the other hand, the highly refrangible or ultra violet rays of the spectrum stimulate growth of a spindling- nature, which would be undesirable to most crops.
Herve-Mangon was one of the first to demon strate that electric light was capable of producing chlorophyll in plants as well as inducing heliotrop ism, and as far back as 1869 Prillieux showed that electric light is capable of promoting assimilation.
The first recorded horticultural experiments with electric light were made by Dr. C. W. Siemens, an English physicist. He experimented with a variety of plants, such as strawberries, tomatoes, grapes and melons, and found that an arc light produced decided effects on the growth of these crops, sometimes producing beneficial, and, at other times, injurious effects. He ascertained very early in his experiments that a naked or unscreened light was injurious at short range, but that the inter position of a glass globe or ordinary window-pane prevented such injury. He demonstrated that an arc light could be placed over a greenhouse with good results, the glass in such cases screening off the injurious rays, and found that plants developed earlier under screened lights than otherwise. As a result of his experiments he became very sanguine that electric light could be used to advantage in horticulture, and he was the first to employ the term " electro-horticulture " to designate this new appli cation of electrical energy. He showed that growth can be hastened by the addition of electric light to daylight, and that injury does not necessarily fol low continuous light through the twenty-four hours ; that electric light often intensifies the green color of leaves, producing a deeper color in flowers and modifying the flavor of fruits. Siemans maintained that the addition of electric light enables plants to stand a higher temperature in a greenhouse.