116 PROTECTION OF POLLEN. 



weather the ligulate florets and bracts stand out in rays from the periphery of 

 the capitulum, but in bad weather and at night they are raised and actually 

 bent over the central tubular florets. They are either disposed so as to form 

 together a hollow cone over these florets, or else they overlap one another like 

 the tiles on a roof ; often, too, they are twisted together in apparent disorder into 

 a tuft, but they are always so arranged as to afford complete shelter to the central 

 florets and to the pollen exposed by them. 



It is a remarkable fact that the length of these incurving rays stands in a definite 

 relation to the diameter of the capitulum. Heads with large discs and great 

 numbers of tubular florets have relatively long marginal rays, those with small 

 discs and few tubular florets have relatively short rays. Moreover, at first when 

 the florets in the middle of the disc are still closed, and only the tubular florets set 

 near the margin have extruded their pollen, the ligulate florets of the ray and 

 the radiating bracts are still short because they only have to shelter their nearest 

 neighbours; but as soon as the flowers in the middle of the disc open, the 

 peripheral florets lengthen so as to be able to cover them also. Thus the roof here 

 actually grows in proportion to the dimensions of the surface requiring shelter. 



The changes affecting the position of petals, ligulate florets, and bracts, which 

 have been briefly described and which are classed together under the name of 

 closing movements, take place in most plants in from thirty to fifty minutes, but 

 in a few cases they are much more rapid. Sometimes the process of closing is com- 

 pleted in the course of a few minutes. With Alpine plants it may happen that the 

 flowers shut and open several times within an hour. The warmth imparted by a 

 casual ray of sunshine is sufficient to cause the flowers of Gentiana nivalis to 

 spread out their deep -blue petals, but no sooner does the sun disappear behind a 

 cloud than the petals wind themselves round one another in a spiral and close up, 

 forming a hollow cone. If the sun comes out again the corolla is once more open 

 in the course of a few minutes. 



In plants with funnel-shaped, tubular, or bowl-shaped corollas, as, for example, 

 the Thorn-apple, Gentians, and the Venus' Looking-Glass {Datura, Gentiana, Specu- 

 laria), the phenomenon of closing is attended by a complex folding, bending, and 

 twisting of the petals; but as a rule the position assumed by the petals on such 

 occasions is the same as that which they previously exhibited in the bud. Generally 

 speaking, most flowers and heads of flowers when closed at night have the same 

 appearance as they had in the bud state. 



For the proximate cause of the movements of closing we must undoubtedly 

 look to alterations in the tension of the layers of tissue involved in the operation. 

 These alterations are due chiefly to variations of heat and light. Fluctuations in 

 the degree of moisture of the air may also partly contribute to the result. In 

 the Carline Thistle (Carlina acaulis), indeed, the opening and closing of the 

 heads depends solely on this condition, and temperature is only a factor inasmuch 

 as the relative moisture of the air is generally diminished as the heat increases 

 in the parts of the world where the plant grows. Owing to this property of 



