DEVELOPMENT OF PLANTS 



481 



The contact of the insect acts as a stimulus, causing the leaves 

 to roll up and so bringing to bear upon him a larger surface of 

 digestive glands. One of the species of Pinguicula is used in 

 northern countries to curdle milk in place of "rennet" which 

 contains the similar digestive fluids of calves' stomachs. 



The potato family (Solanaceae) shows many of the irregu- 

 larities of the figworts, but the majority of its 1,700 species have 

 regular flowers as illustrated in Fig. 334. While the family 





Fig. 334. Flower of the potato {Solanum tuberosum), showing the stamens 

 inserted on the tube of the corolla and encircling the style. 



is largely tropical, many of the species are cultivated, as the 

 potato, eggplant, tomato, cayenne pepper (Capsicum). Poison- 

 ous, acrid and narcotic properties are characteristic features of 

 the family. Belladonna and atropine from Atropa, stramonium 

 from Datura and nicotine from Nicotiana tahacum are character- 

 istic drugs. Ground cherry (Physalis), nightshade {Solanum), 

 Petunia, etc., are cultivated forms. 



153. A Transitional Order.- — It would appear that all the 

 changes possible in the hypogynous type of flowers had been 

 wrought in the members of the Polemoniales and that the next 

 step in advance must be to the epigynous flower. This is seen 

 to be the case with the madder order, Rubiales. Here the 

 flowers are epigynous and the parts are generally in fives though 

 the carpels vary greatly in number. The simplicity in the struc- 

 ture of the flower and often also the form of the inflorescence, 

 as in the elderberry {Samhucus), bedstraw {Galium) and arrow- 

 wood ( Viburnum) is strikingly suggestive of the Umbellales 

 (Figs. 335, A, B; 324). It appears that each step in advance 

 is attained in a very simple way and that only gradually are 

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