724 SUMMARY OF CURRENT RESEARCHES RELATING TO 



BOTANY. 



GENERAL, 



Including- the Anatomy and Physiology of Seed Plants. 



Structure and Development. 

 Veg-etative. 



Xerophytic Adaptations of Leaf-structure.* — J. F. McClendon 



gives an account of his studies of the leaf-structure in Agave, Hesperake, 

 Dasylirlon, Nolina, aud Yucca. The simplest type of stoma occurs in 

 Yucca aloifolia : the guard-cells are sunk but little, and are reached by 

 an air-passage which pierces the thick epidermis. Beneath the stoma is 

 a system of air-cavities, the upper part being lined with cutin. In 

 Agave yueccefolia the air-passage is shorter, and there is less cutin in 

 the air-cavities. The genera examined show a gradual increase in the 

 complexity of the supra-stomal air-passage. In addition to being sunk 

 below the epidermis, the stomata are often placed in grooves which 

 close over them aud prevent transpiration. The stomata in this position 

 have less complex air-passages than are found in the more exposed ones, 

 but have more effective arrangement of air-passages to promote rapid 

 respiration of the deeper tissues. The leaves of Yucca and its allies 

 have a thick cutinised epidermis, and the vascular bundles and internal 

 mechanical tissues are arranged with special reference to protection of 

 the stomata. The shape and method of protection of the stomata 

 vary according to habitat. The above adaptations to a xerophytic 

 habitat disappeared when the plants were placed under conditions of 

 increased moisture and diminished sunlight. 



i &" 



Extra-floral nectaries. | — K. Ono has examined a number of plants 

 with the view of throwiug light upon the anatomy and physiology of 

 extra-floral nectaries. The author finds that there are two forms of 

 such nectaries, one being represented by Polygonum sachalinense, and 

 the other by Prunus yedomsis. The first type is epidermal in origin, 

 while the second develops from both epidermis and hypodermis. When 

 these nectaries occur on leaves, they are situated on the under surface, 

 but when on petioles they are on the upper surface. They consist of 

 true secretory glands on the surface, and subglandular cells of indirect 

 importance. External conditions, of which moisture appears to be most 

 important, are of small importance relative to internal conditions. 

 Epidermal nectaries do not secrete so actively as do those derived from 

 hypodermis and epidermis. These nectaries are attractive to ants. 



* Amer. Nat., xlii. (1908) pp. 308-16 (25 figs.). 



t Journ. Coll. Sci. Tokio, Japan, xxiii. (1907) pp. 1-28 (3 pis.). 



