164 REPRODUCTION 



Gardeners use vegetative reproduction by making cuttings 

 or by layering, which means pegging down below the soil 

 portions of shoots which have been partially cut through. 

 Carnations, mulberries and many other plants are often 

 reproduced in this fashion. 



Vegetative reproduction is particularly common amongst 

 Alpine plants, probably because the short season and low 

 temperatures are unfavourable to the formation of flowers 

 and fruits. Vegetative reproduction is not common among the 

 Gymnosperms (firs, larches, pines), but in ferns, club-mosses, 

 horsetails, etc. it is very prevalent. It is so common amongst 

 mosses that some kinds have never been seen in fruit. It is 

 equally common amongst the Algae and the Fungi. In certain 

 cultivated plants it seems to be the sole method of repro- 

 duction. The pine-apple and the banana are often seedless. 

 Sugar-canes rarely flower and the Jerusalem artichoke has 

 been grown in our gardens since the time of Queen Anne 

 solely from tubers. The fundamental difference between 

 vegetative and sexual reproduction is that organisms which 

 are propagated in the former way keep true to type. When 

 you have a good strain you can maintain it; but it does not 

 make for variety. 



Vegetative reproduction is much less common amongst 

 animals than plants. In the latter, as we have seen, it is fully 

 maintained even in the highest plants; but in animals it 

 does not get very high up in the scale. Many animals 

 reproduce by dividing, for instance hydroids such as 

 Hydra, Should the products of the division separate com- 

 pletely, we have two hydroids instead of one; but in many 

 cases the results of the division remain in continuity and then 

 we have a colony, and each constituent forms what is called 

 a zooid. Even animals as high in the scale as sea-anemones 

 \\^11 divide into two, and the same is true of many worms. If 

 an earthworm be accidentally divided it will regenerate the 

 cut surfaces and form two earthworms; but many aquatic 

 worms divide normally. In the case of the Palolo-worm 

 mentioned on page 145 the hinder end is packed mth repro- 

 ductive cells. This separates off and floats to the surface. The 

 front half, which remains in the depths of the coral rock, then 



