124 PROTECTION OF POLLEN. 



is due the security enjoyed by the pollen developed from their tissues. For 

 if dewy nights or wet weather occur after dehiscence has taken place and 

 whilst the pollen is exposed at the apertures in the anther-cavities, the latter 

 close up again and encase the pollen once more. The mature pollen is then 

 protected from wet just as effectually as it was during the period of its 

 maturation, for no injurious effect can be exercised by rain or dew through 

 the walls of the anther upon the pollen-cells concealed within. When there is 

 a return of warm, dry weather the anthers open afresh in the same manner 

 as on the occasion of their first dehiscence. Precisely the same processes as 

 were described on pp. 91-93 are repeated. If the anthers are unilocular with 

 transverse dehiscence, like those of Globularia and the Lady's Mantle (Alche- 

 milla', see figs. 226 5,6,7,8,9,10^ ^ e su tures open and shut like lips. If the 

 dehiscence is opercular, as in the Bay Laurel (Laurus nobilis\ see figs. 

 226 n> 12> 13> 14 ), the valves shut down again and force the pollen adherent to 

 them back into the open recesses of the anthers. Lastly, if the dehiscence is 

 longitudinal and the anther- walls open outwards like folding doors and at the 

 same time become revolute, as in Thesium and Bulbocodium (cf. figs. 226 1>2 ' 3 ' 4 ), 

 the movement is reversed in wet weather, and the two valves close completely 

 together again. 



In the Arctic regions and amongst the mountains of Central Europe where 

 copious deposits of moisture occur during the flowering season common to most 

 plants, the number of species possessing anthers which open and shut periodically 

 is not great. Besides those already named, i.e. Bulbocodium, Thesium, and the 

 Alchemilla, only the Plantains (Plantago) and Ranunculacese, especially those 

 with pendulous anthers (Thalictrum), remain to be mentioned as exhibiting this 

 phenomenon particularly clearly. It appears to be much commoner in warmer 

 parts, especially in sub-tropical and tropical regions ; at all events, this periodic 

 opening and closing of the anthers is exhibited to perfection in the following 

 plants: Cinnamon-trees, the Camphor- tree, the Laurel and Lauraceous plants 

 generally, Araliacese and Cycadeae, the various species of Ricinus and Euphorbia, 

 Cistus, the Vine (Vitis), and indeed the majority of AmpelideaB, the Tulip-tree 

 and Magnolias (Liriodendron, Magnolia), and lastly, amongst Conifers the genus 

 Cephalotaxus. 



The phenomenon in question is the result of changes in the condition of the 

 air in respect of moisture, and depends upon the contraction and expansion of 

 the hygroscopic cells which we noticed in the last chapter as being developed 

 underneath the epidermis of the anther-walls. As in the case of the movements 

 of the involucral bracts on the capitula of the Carline Thistle, the process is only 

 affected by heat inasmuch as the relative degree of moisture in the air alters with 

 a rise or fall of temperature. Seeing that under ordinary conditions variations 

 of temperature and increase or decrease of humidity are connected with the 

 alternation of day and night, it is clear that a periodicity will also be manifest 

 in the opening and closing of anthers, and that in the evening when the degree 



