PRACTICAL WORK 373 



strong selective effect which will inevitably lead to the 

 prevalence of these species in this particular habitat. 

 The same general consideration holds of most of 

 these so-called " adaptive " characters. They have not 

 in the first instance appeared in relation to the conditions 

 in which they are of special use to the organism, but 

 the existence of the character may favour the spread 

 of the species when these conditions arise^ and may 

 even in extreme cases secure its continued existence 

 in circumstances which would lead to the extihction 

 of a species not so equipped. 



PRACTICAL WORK. 



Development of Seed and Embryo. 



(i) Slit open the fresh ovaries of Shepherd's Purse {Capsella), 

 choosing those from which the petals have just fallen, and older 

 stages up to the full-sized capsules. Draw under the low power 

 seeds of different stages of development. 



Squeeze the ovules of different ages under a coverslip so as to 

 press out the embryos, and draw various stages in the develop- 

 ment of the embryo. Note the suspensor and the spherical 

 embryo in the young stage, and the subsequent development 

 of the two cotyledons. 



Dehiscent Fruits. 



(2) Examine the young fruit of the Sweet Pea (Lathyrus) 

 projecting from the remains of the flower. Split the ovary 

 down and note the two rows of young seeds attached to the 

 ilpper edge. Note the similarity of the single carpel here with 

 the carpel of Caltha (p. 359). In the ripe fruit note the mature 

 seeds and the mode of dehiscence. 



(3) Examine any available examples of syncarpous, dry 

 dehiscent fruits (capsules) . 



Indehiscent Dry One-Seeded Fruits. 



(4) Cut the young " aggregate fruits " of Herb-bennet (Geum) 

 longitudinally in half so as to show the insertion of the individual 

 achenes on the receptacle. Note that a hook is developed in 

 the style just below the stigma, which falls off. 



