PLANT PHYSIOLOGY 309 



while at night they droop and usually fold together (see 

 Plant Relations, pp. 9, 10). These are the so-called nycti- 

 tropic movements or " night movements," which maybe ob- 

 served in many of the Legumes, as clover, locust, bean, etc. 

 In still other cases, mechanical irritation induces move- 

 ment, as sudden contact, heat, injury, etc. Some of the 

 " carnivorous plants " are notable illustrations of this, es- 

 pecially Bionwa, which snaps its leaves shut like a steel 

 trap when touched (see Plant Relations, p. 161). Among 

 the most irritable of plants are the so-called "sensitive 

 plants," species of Mimosa, Acacia, etc., all of them Le- 

 gumes. The most commonly cultivated sensitive plant is 

 Mimosa pitdica (Fig. 280), whose sensitiveness to contact 

 and rapidity of response are remarkable (see Plant Rela- 

 tio?is, p. 48). 



REPRODUCTION 



172. Reproduction. — The important function of repro 

 duction has been considered in connection with the various 

 plant groups. Among the lowest plants the only method 

 of reproduction is cell division, which in the complex 

 forms results in growth. In the more complex plants va- 

 rious outgrowths or portions of the body, as gemmae, buds, 

 bulbs, tubers, various branch modifications, etc., furnish 

 means of propagation. All of these methods are included 

 under the head of vegetative multiplication, as the plants 

 are propagated by ordinary vegetative tissues. 



When a special cell is organized for reproduction, dis- 

 tinct from the vegetative cells, it is called a spore, and re- 

 production by spores is introduced. The first spores devel- 

 oped seem to have been those produced by the division of 

 the contents of a mother cell, and are called asexual spores. 

 These spores are scattered in various ways — by swimming 

 (zoospores), by floating, by the wind, by insects. 



Another type of spore is the sexual spore, formed by 

 the union of two sexual cells called gametes. The gametes 



