2] UPON THE DIRECTION OF GROWTH 339 



all directions at first, but quickly grow towards the pistiL 

 (Fig. 92, c). MIYOSHI ('94 a ) found that the top of the pistil was 

 most attractive and that lower sections were less so until the 

 ovary is nearly reached, when the attraction is high again. If 

 the ovules of S cilia are placed on the plate of agar with its own 

 pollen, the pollen-tube will even grow into the micropyle of 

 the ovule. Pollen-tubes of a different species or even genus, 

 e.g. Diervilla rosea, Ranunculus acer, etc., may likewise enter 

 the Scilla ovule, and even hyphae of Mucor stolonifer will turn 

 towards the ovules and penetrate into them. Thus the attract- 

 ing stuff is not a specific stimulant confined in its activity to 

 one kind of pollen-tube nor even to pollen-tubes in general. 



The nature of the attracting substance has been studied by 

 MIYOSHI. Glucose is certainly present in the fluid excreted 

 by the ripe stigma of many phanerogams ; and, if the agar-agar 

 substratum contain a 2% solution of cane sugar, there is no 

 longer chemotropism with reference to the ovule, since now the 

 attraction is not confined to a particular point. So it may be 

 concluded that it is the sugar of the ovule or stigma which 

 attracts the pollen-tube and that the excreted fluid contains 

 about a 2% solution. Consequently, the chemotropism of 

 pollen-tubes may be only a special case of chemotropism to 

 sugar. 



Further experimentation confirmed MIYOSHI ('94) in this 

 conclusion. Using the method employed by him in the case of 

 hyphse (p. 340), he injected Tradescantia leaves with various 

 solutions and sowed pollen of Digitalis purpurea upon the 

 leaves. When pure water was injected there was no effect, 

 but the substances named in the following list attracted the 

 pollen-tubes so that they grew down through the stomata into 

 the leaf : 



cane sugar . 2-8% levulose . 1% (slight action) 

 grape sugar, 4-8% lactose . 1-2% ("slight") 

 dextrin . . 1-2% 



The following solutions were neutral : 



maltose 1% asparagin ... 2% 



meat extract . . . glycerine . . . "2-5% 



peptone gum arabic . . 2% 



