TISSUES AND SIMPLE ORGANS. 63 



absent or reduced to a minimum, in which case the stem-parenchyma 

 alone carries on the process of assimilation; other means are the 

 rolling up, curling, or folding of the leaf surfaces, the vertical 

 position of leaf-blades, and the formation of mucilaginous substances 

 in the epidermis for the purpose of retaining moisture. Later, in 

 the discussion of the aerating system, we will learn to know another 

 characteristic phenomenon occurring in various forms which has to 

 do with the position and structure of stomp.ta (the openings of the 

 aerating system). This phenomenon also belongs to the above- 

 enumerated arrangements for reducing the loss of moisture. 



The mention of trichomes made above lead me to make the 

 following statement. The anatomy of trichomatic organs has been 

 accurately studied ; their physiological significance is, however, not 

 correspondingly well known. For that reason I shall conclude this 

 chapter rather hurriedly. Of the great variety of forms of trichomes 

 I shall select only a few represented in Fig. 37. If a glandular hair 

 secretes an ethereal oil, its function seems clear, namely, to attract 

 insects which will carry the pollen. If the secretion is of a sticky 

 consistency it evidently serves to keep off injurious crawling in- 

 sects, since these take the honey without aiding in cross-fertiliza- 

 tion. The flattened or shieldlike trichomes which cover the breath- 

 ing-pores evidently serve to guard against excessive loss of moisture. 

 The satinlike shimmer of floral leaves is due to papillose trichomes 

 (conical projecting epidermal cells). In some instances it has been 

 proven that trichomes with thin-walled areas near the base serve to 

 admit moisture (rain, dew). Still a considerable number of trichome- 

 structures remain whose physiological significance is not satisfac- 

 torily explained. 



III. FUNCTION OF MECHANICAL TISSUES. 



Even a superficial consideration of the plant kingdom suffices to 

 teach that the mechaniGol influences surrounding land-plants, water- 

 plants, aerial organs, subterranean organs, etc., are different, and 

 that these various plants and plant-organs require definite adaptations 

 as to the firmness of the tissues concerned in order that they (as the 

 normal course of things teaches) may be maintained in their entirety. 



In upright stems — in fact, in all organs which must maintain 

 themselves in an upright or in Vi free horizontal position — bending 

 enters into consideration, especially as the result of air currents; also 



