126 Practical Plant Biology. 



of one another and each forms a separate plant. This forms 

 another example of the frequent though at first sight paradoxical 

 connection of death with reproduction. 



The second method of asexual reproduction is also very pro- 

 lific. It is carried on by means of gemmae, which are small lens- 

 shaped masses of cells about a millimetre in diameter, and when 

 detached from the plant grow into new plants. These gemmae 

 are found attached to the bottom of small cup-shaped organs 

 (cupules) developed on the middle of the thallus. .The cupules 

 have a colourless serrated margin. Each gemma is attached to 

 the bottom of the cupule by a short stalk. It arises as a two- 

 celled hair. The distal cell enlarges and subdivides and thus 

 produces a lens-shaped mass of cells. There are two indenta- 

 tions in the periphery diametrically opposite to one another. 

 These become the first growing points of the young plant. When 

 the gemma has attained its full size it is set free by the breaking 

 of the stalk formed of the basal cell, and if it finds itself in a suit- 

 able position, certain of its superficial cells grow out and develop 

 into rhizoids. The rhizoids develop from specially large cells of 

 the gemma which do not possess chlorophyll. They are found on 

 both sides, but only those develop which have a chance of being 

 of use to the young plant. This purposeful development has 

 been shown to be due to the response of these cells to three kinds 

 of stimuli. Thus moist air and a positive gravitational stimulus 

 evoke their development, while light and a negative gravitational 

 stimulus inhibit it. 



The response then to these stimuli secures that the rhizoids of 

 the young plant are normally developed on that side which is in 

 contact with the ground. As soon as these rhizoids begin their 

 absorptive functions the growing points in the two depressions on 

 the margin of the gemma become active and they soon convert 

 the gemma into a small thallus. At first there is no differentia- 

 tion in the gemma into dorsal and ventral aspects, either side can 

 develop according as external stimuli direct its growth : but soon 

 a certain point is reached in the development when the dorsi- 

 ventrality of the tissues of the young plant is established and then 

 the dorsal side loses the power of developing rhizoids, etc., like 

 the ventral aspect, and the ventral aspect loses the power of 

 developing as the dorsal. The cells of the gemma, which have 

 developed in a vertical plane while in the cupule, are not differ- 

 entiated in their responses to these stimuli, but those formed from 

 the growing regions, when the gemma is in a horizontal position, 

 respond differently to the same stimuli. Hence we must assume 



