CHLOROPHYLL AND LIGHT INTENSITY. 393 



rays. Such an arrangement is found, for example, in the leaves of the aromatic 

 Satureja hortensis, originally growing wild in the Mediterranean floral district, and 

 cultivated in gardens under the name of Summer Savory, of which leaves a small 

 portion is represented in cross section on page 139, figure 25 a, q. Before the 

 sunbeam reaches the chlorophyll-granules of the green cells in the middle of the 

 leaf, it must pass through these epidermal cells filled with violet sap, and here it 

 becomes so weakened and also so changed that an injurious influence on the 

 chlorophyll-granules is out of the question. We must not omit to notice here that 

 the violet light-reducing colouring-matter in the epidermal cells is more abundantly 

 developed the intenser the light to which the plants in question are exposed. If 

 plants of the Summer Savory grow in shady places, their leaves remain green on 

 the upper sides, and scarcely any traces of the violet colouring-matter are to be 

 discovered in the epidermal cells. If, on the other hand, they have germinated in 

 shadeless districts, both stem and leaves are coloured dark violet, and the cell-sap in 

 the epidermal cells is then of a deep tint (see fig. 25 A, q on page 139). Some years 

 ago I cultivated seeds of the Summer Savory in my experimental garden at a height 

 of 2195 metres above the sea-level in the Tyrol. As is known, the sun's rays are 

 much more powerful in the Alpine heights than in the valley, and it was therefore, 

 indeed, to be expected that the leaves of the germinating plants would be of a much 

 darker tint than in the shadeless gardens of the valley below. In fact, the colouring- 

 matter developed in extraordinary abundance; even the stems and leaves actually 

 became a dark brownish violet. It is, therefore, beyond question that the quantity 

 of colouring-matter in the epidermal cells directly exposed to the sun increases 

 with the increase of the intensity of the light. Obviously this protection of the 

 chlorophyll can only occur when the plants possess the materials for forming the 

 pink colouring-matter in their green organs. When this is not possible, when the 

 characteristic constitution of the protoplasm does not permit the development of 

 the colouring-matter named in the foliage-leaves, the chlorophyll must be pro- 

 tected against the glaring light in another way, and if the plant species is not 

 able to ward ofl" the over-abundance of sunlight in the new position, it perishes 

 entirely. Flax (Linum usitatissimum) was sown next to the Summer Savory in 

 the Alpine experimental garden — a plant which bears the direct sunlight quite 

 well, and flourishes in the valley as well as in the plains in sunny situations. 

 However, the light of the Alpine region was too brilliant for the germinating flax- 

 plants; the leaves turned yellow, their chlorophyll was destroyed, and the seedlings 

 became pale and perished. Flax has not the capacity of manufacturing the 

 colouring-matter in its superficial cells, and it is also not organized to produce 

 covering hairs on the leaves and stem, or to thicken its cuticular strata suitably — 

 in a word, to adapt itself to the position and to provide itself, under the increased 

 light intensity, with corresponding sun-shades and light-extinguishers. While close 

 at hand, the Summer Savory, which requires just as much warmth, and an equally 

 long vegetative period as flax, reached the flowering stage, and even produced ripe 

 fruits capable of germinating, the flax died before the development of its flowers. 



