On the Movements of Petals 17 



gescence as the cause. Farmer gives to a localized tissue on the 

 outer face of Tulipa petals the power of causing the perianth 

 to open as a result of irritation. He says the opening is, how- 

 ever, due to the stimulus (he does not state what, but denies 

 cpinastic and hyponastic growth here) affecting the protoplasm 

 and producing movement. Sections that I have made through 

 the nerves of the corolla of Mirabilis do not show the differences 

 in structure, indicated by Dutrochet, sufficient to cause opening 

 and closing. He states that the cellular tissue on the outside 

 would tend to curve out by filling with water in excess — opening, 

 the fibrous tissue on the inner side tending to curve in by oxida- 

 tion — closing. Cross-sections through the nerves of the corolla 

 of Mirabilis show about this proportion of the dift"erent tissues: 

 145 /a of parenchyma on the outside of the nerve, exclusive of 

 epidermis, y2 /u, of fibrous tissue, and on the inner side, 102 /x, of 

 parenchyma, exclusive of the epidermis. We could scarcely ex- 

 pect movements such as Dutrochet speaks of to occur as results 

 of turgescence of the parenchymatous tissue when it is present 

 on both sides of the fibrous tissue and in such proportions. He 

 states also that the morning out-curving of parenchymatous tis- 

 sue (opening) of the dandelion is brought about by a strong 

 rise of sap under the influence of light, causing turgescence. 

 The closing, he says, is due to the diminution of this force, and 

 to the gradual filling of the fibrous tissue with oxygen during 

 the day. This could not occur, since the osmotic pressure of the 

 cell sap is increased only 5 per cent with a rise in temperature 

 of 15° C, and the temperature never need change this much to 

 induce closing; hence, some other cause must be sought. 



HEMERANTHOUS TYPE 



Experiments zvith Taraxacum taraxacum. The flowers of the 

 dandelion open from 7:00 to 8:00 a.m. in the greenhouse or in 

 the open in May, and close from 5:00 to 6:00 p.m. On cool, 

 cloudy days they may not open at all, and in a sudden lowering 

 of the temperature, usually accompanied by cloudiness, as upon 

 the approach of a storm, they may close at once. Temperature 



207 



