PROTECTION OF POLLEN. 127 



tant provision at all events consists in the fact that the injurious effect of rain 

 or dew on the pollen-cells may be obviated by certain special sculpturings on 

 the surfaces of these cells. Reference has already been made to such cases at 

 the conclusion of the last chapter. They are on the whole rare, and are limited 

 apparently to plants of the tropical and sub-tropical regions. The pollen of the 

 beautiful climbing Cohoea scandens (cf. fig. 217 i), one of the Polemoniaceae, will 

 serve as an example. On the surface of this pollen may be observed a number 

 of little pits with angular rims which make it look at first sight almost like a 

 honey-comb. The pits are not, it is true, so deep as those of a honey-comb, 

 but they are deep enough to prevent the air with which they are filled from 

 being displaced by water dropping upon the pollen. Thus air remains in the 

 pits and thereby affords protection from wet, for it forms an intermediate layer 

 separating the thin parts of the cell-membrane from the water. The thick 

 layers of the cell-membrane which project in ridges are still liable to be wetted, 

 but water cannot penetrate at once through them into the interior of the cell, 

 and such an entrance it is that constitutes the greatest danger to the pollen. 

 A gradual absorption of watery liquid — especially that which is derived from 

 the cells of the stigma — is not only not avoided, but is even necessary for the 

 subsequent development of the pollen-cells. 



The instances chosen hitherto for the exemplification of the numerous contriv- 

 ances whereby the pollen in flowers is protected against wet belong, for the most 

 part, to the category of those which have developed one form of protective apparatus 

 only. Frequently, however, two or even three methods of defence co-exist, so that 

 in case one contrivance should fail there is another in reserve. This occurs in cases 

 where the plant has only a meagre stock of pollen, where the number of flowers on 

 one individual and the quantity of pollen-cells produced from each flower are small, 

 and therefore there is not much pollen to waste, where the time allotted to a plant 

 in which to unfold all its flowers is extremely limited, and where the transport of 

 the pollen from flower to flower is accomplished exclusively by flying insects, whose 

 visits are sometimes delayed for several days when the weather is unfavourable. To 

 mention a few instances with more than one means of protection, in many Anemones 

 and Crow-foots, the Hepatica, the Rock-rose, and the Wood-sorrel (Anemone, 

 Ranunculus, Hepatica, Helianthemum, Oxalis), not only do the petals close over 

 the pollen-laden anthers, but the flower-stalks also bend, causing the flowers to nod. 

 In the Daisy (Bellis), the Corn Sow-thistle (Sonchus arvensis) and many other Com- 

 posites not only do the ligulate florets of the ray incline towards one another and 

 form a roof over the pollen of the central florets in cloudy weather and in the 

 evening, but in addition the peduncles become bent or pendent. In Podophyllum 

 peltatum the pollen is sheltered by the bell-shaped flower, but in addition to this the 

 peltate foliage-leaves are also spread out over the flowers and act as umbrellas. The 

 synchronous closing of both anthers and petals over the pollen when rain threatens 

 is a phenomenon that may be easily observed in a number of plants, as, for 

 instance, in Bulhocodium (cf. figs. 226 ^' ^' ^' *•). 



