372 SUMMARY OF CURRENT RESEARCHES RELATING TO 



thickened. They are never entirely homogeneous, but are furnished 

 with a larger or smaller number of dots, which in some cases always 

 remain closed, as in Vascular Cryptogams, in others are at an early 

 period covered with callose substance, being shortly afterwards 

 changed into a true sieve by the appearance of numerous perforations, 

 as in Phanerogams. The mature sieve-tube never contains a nucleus, 

 having only a thin parietal layer of jjrotoplasm which marks its 

 vitality, and which entirely disappears with the death of the organ on 

 the cessation of the life of the sieve-tube. 



Structure and Functions of Stomata.* — A. Tschirch distinguishes 

 the following distinct parts of the stomatal apparatus : — (1) The 

 eisodial opening, or opening into the anterior chamber; (2) The 

 opisthodial opening, or opening into the posterior chamber ; (3) The 

 central fissure ; the true fissure between the two openings, which 

 sei)arates the guard-cells ; (4) The outer and inner cuticular ridges, 

 which are often circular and surround the two openings ; (5) The 

 outer stoma, or outer space which causes the depression in depressed 

 stomata; (6) The circular wall, or margin which projects above the 

 epidermis when the depression is pitcher-shaj)ed ; (7) The circular 

 ridges when the stoma is funnel-shaped ; and (8) The epidermal 

 opening, or actual orifice of the outer stoma. 



Tiie author classifies the different forms of stomatal apparatus 

 under eighteen types, which are described in detail. 



Excessive evaporation is prevented, firstly by the form of the 

 stomata themselves ; and secondly, by various special contrivances for 

 the purpose, as the structure of the epidermis and accumulation in it 

 of incrustation of calcium oxalate ; coatings of wax on the epidermis ; 

 haii-y formations ; limitation of the large intercellular spaces in the 

 merenchyma of the leaf ; the nature of the cell-sap ; and the form 

 and (frequently vertical) position of the leaves. 



The number of stomata on a unit of surface in nearly related 

 plants is larger in those which grow in moist, smaller in those 

 which grow in dry habitats. The arrangement of the stomata is 

 associated with purposes of protection. In those leaves which roll 

 up when dry, the stomata on the concave side are enclosed ; and the 

 same is the case in grasses which grow in dry situations. 



Stomata of Stapelia.t — J. Jako describes the structure of the 

 comiDlicated stomata of Stapelia variegata and trifida, which presents 

 considerable analogy to that in many Monocotyledons, especially 

 Tradescantia and Commelyna. 



Influences of External Forces on the Direction of Growth.^ 



— Sachs attributes the direction taken by pollen-tubes, after the 

 grains attach themselves to the stigmatic papillae, down the style, to 

 the arrest of growth on the side in contact with the solid substance ; 

 Darwin, to the endeavour to avoid the light. L. Kny has attempted 



* Linnsea, xliii. (1881) pp. 139-252 (1 pL). 

 t Jlagyar Nove'nyt. Lapok, v. (1881) pp. 151-6. 



j SB. Bot. Vcreins Prov. Braudeuburg, xxiii. (1881). See Bot. Centiulbl., ix. 

 (1882) p. 10. 



