The Light Relation 429 



This effect is of much practical value in the forcing of rhu- 

 barb for early market (Fig. 87). 



The leaves of tobacco produced in the shade tent are not 

 only larger, but thinner, and they possess relatively larger 

 air spaces and more spongy parenchyma ; the fibrovascu- 

 lar bundles, or venation systems, are less prominent, and 

 the leaf is thereby improved for wrapper purposes. In the 

 fibrovascular bundles of half-shade plants the mechanical 

 supporting tissues are usually reduced, and this is a factor 

 in blanched celery. Self-shading to produce crispness and 

 tenderness may be practiced in some cases; thus in the 

 cultivation of Cose lettuce, romaine, and cauliflower, 

 the simple operation of bringing together and tying the 

 leaves in the form of a head may produce the effect 

 desired. 



261. Half-shade and quality. Plants grown in half- 

 shade commonly contain a higher per cent of moisture and 

 less ash. It has been ascertained that the apparent acid- 

 ity of strawberries is increased by shade. This apparent 

 increase is, however, due to lessened accumulation of 

 sugar in the berries. 



The aromatic products of plants are not important as 

 animal nutrients, but they are physiologically essential, 

 and represent almost the sole value of many economic 

 plants used as condiments. In 1838, De Candolle called 

 attention to the diminished production of savors and odors 

 in shaded plants. It was found later that plants removed 

 from southern latitudes to the latitude of Scandinavia 

 during the two months of maximum sunshine in the latter 

 region, showed an increase in the development of aromatic 

 products. Indeed, it has long been suggested that many 



