ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 47 



round the margin. Numerous stomata are found on both Bides of the 

 leaves, and are overarched by small epidermal cells, among which are 

 scattered capitate hairs ; the cuticle is very thick. The mesophyU is 

 composed of a double layer of palisade-cells separated by a layer of 

 irregular rounded cells ; there ace no intercellular spaces except beneath 

 the stomata. Embedded in the mesophyU are numerous scleroblasts, 

 either simple and columnar or large branched structures, usually confined 

 to the outer palisade-layer, through which the branches extend until they 

 reach the inner wall of the epidermis, beneath which they expand. The 

 leaves of plants grown in the shade differ greatly from those grown in 

 the open. The leaf itself is thinner, the cuticle is thin and transparent, 

 while both epidermal and palisade-cells are considerably shorter. There 

 are fewer hairs and stomata, and the structure of the latter is less com- 

 plex. Scleroblasts are rare, w T hile the mesophyU is irregular and has 

 intercellular spaces. The difference in structure is apparently an adapta- 

 tion to xerophytic conditions. The thick dark-coloured cuticle, the 

 regular and closely-packed mesophyll-cells with the accompanying 

 scleroblasts, appear to have the same function, viz. to diminish trans- 

 piration. The sclerenchymatous fibres of the vascular bundles give 

 rigidity to the leaf and also assist in the reduction of transpiration. 



Reproductive. 



Mechanism of Anther-dehiscence.*— M. Schips has studied the 

 causes of the opening of anthers, and shows that the results of his ex- 

 periments are in opposition to the theory supported by Hannig and 

 other writers, according to which the opening is caused by contraction 

 resulting from cohesion. In the first part of the work the author dis- 

 cusses the theory in relation to his own experiments dealing with 

 (1) the air-contents of the anthers at the time of opening and the 

 opening in moist air ; (2) the formation of folds in thin membrane ; and 

 (3) the method of bringing about the opening by liquids capable of 

 removing water ; in each case it is maintained that the majority of the 

 results are in conflict with the " cohesion theory." In the second part 

 the author deals with the hygroscopic mechanism, and expresses the 

 opinion that the shrinking of the thickened fibres is insufficient to cause- 

 opening. The present review by E. Hannig criticizes the unsatisfactory 

 methods of experiments described and their untrustworthy results. 



Delayed Germination of Alisma.t, — W. Crocker and W. E. Davis 

 have studied the germination of Alisma Plaatago, with the following 

 results : Dormancy of the seed is due to the mechanical restraint of the 

 seed-coat, which enables the seed to remain in water for years without 

 germination. Neither the w T all of the carpel nor the outer "seed -coat has 

 any influence on dormancy, which is apparently due to the two inner 

 seed-coats. The slightly swollen embryo lies for years within the 

 saturated seed coat, against which it exerts a pressure of about loo 



* Beih. Bot. Centralbl., lte Abt., xxxi. (1913) pp. 1-92. See also Zeitschr. 

 Bot., vi. (1914) pp. 564-7. 



t Bot. Gaz., lviii. (1941) pp. 285-321 (8 figs.). 



