MEANS OF ACCELERATING TRANSPIRATION. 285 



such tenuity, that if the humidity of the air sinks only a few degrees below satura- 

 tion point, or if a transient sunbeam enters the ravine even for a short time, they 

 immediately give off water- vapour. 



Apart from such cases, the exhalation of water- vapour from the superficial cells 

 is scarcely worth noticing; it is almost entirely restricted to the cells of the spongy 

 parenchyma. Here are to be found, indeed, very striking arrangements, which must 

 be regarded as contrivances for increasing transpiration. First of all, where 

 transpiration is to be accelerated, the green, spongy tissue is very strongly 

 developed, the air-containing lacunae and passages, which penetrate the net- work of 

 branched cells like a maze, are enlarged and numerous, and the collective free 

 surface of all the air-bordered cells in the interior of the leaf has a much greater 

 extent than the mere outer surface of the epidermis. In the leaves of many tropical 

 plants which are always surrounded by damp warm air, e.g. in those of the 

 Brazilian Franciscea eximia, of which a section is represented in fig. 62 \ almost 

 the entire thickness is made up of loose wide-meshed spongy parenchyma, and it is 

 evident that water will be exhaled from the cells of this tissue as soon as the 

 temperature of the leaf is raised even to the extent of a few degrees above that of 

 the moist surrounding air by the sunbeams falling upon it. 



In many such plants which urgently require a help to transpiration on account 

 of their situation, the cavities of the spongy parenchyma are extraordinarily enlarged 

 and widened at certain points where the greatest number of stomata are developed. 

 The difference in appearance between such places and other parts of the leaf having 

 dense spongy parenchyma can indeed be recognized by the unaided vision. In such 

 a leaf looked at from above, the large-meshed portions of the spongy parenchyma 

 appear as lighter spots in the dark-green grounding; the leaf is flecked and marked 

 with white. This is not only the case with many plants of damp, tropical forests, 

 but also in those of temperate zones, such as species of the genus Cyclamen, 

 Galeobdolon luteum, the Lungwort (Pulmonaria officinalis), and frequently also in 

 Hepatica triloba, if they grow in very shady places on the damp ground of a forest. 

 It must, of course, not be forgotten that all the white spots and markings of green 

 leaves, which have been named collectively "variegations", are not due to this cause. 

 In those nettle-like plants, known as Bcehmerias, the white spots on the central 

 part of the leaf lamina are caused by peculiar groups of crystals in the epidermal 

 cells, the so-called cystoliths, which reflect the light; in some Piperaceaj they are 

 due to groups of epidermal cells which are filled with air, and below which the 

 palisade cells are absent; in other plants, again, they may be caused by the 

 formation of aqueous tissue, a structure which will be discussed later. In many 

 of those plants with variegated leaves, which are so extensively cultivated for 

 purposes of decoration, the variegation is not normal, but must be considered as 

 pathological, and is in no way connected with transpiration. 



Since, as we know by experience, transpiration of green leaves is increased by 

 light and warmth, it is evidently an advantage for aU those plants to which only a 

 restricted number of sunbeams can obtain access, if their leaf-blades are very large 



