418 SCIENCE PROGRESS 



by a common involucre or envelope of bracts. The calyx of 

 each floret is absent, its place being taken usually by a ring 

 of fine hairs called a " pappus " which crowns the ovary and 

 persists after the death of the floret. In some cases the pappus 

 is inserted directly upon the ovary, 1 as in the Thistle, whilst 

 in others it is raised up on a kind of stalk, as in the Dandelion, 

 its function being the same in both cases ; when the seeds are 

 ripe, the hairs spread out and act like a tiny parachute, bearing 

 the little fruits up on the wind, so that they are often carried 

 over great distances. This peculiar mechanism also ensures 

 that when the seed reaches the ground it shall enter the soil 

 in the way that is most favourable to germination ; its work 

 done, the pappus becomes detached from the fruit and floats 

 away. In other cases, as in the Willowherbs, which sometimes 

 appear as field weeds, the hairs or plumes are different in origin 

 and are attached to the seeds instead of to the fruit but they 

 function in a similar way. 



Other plants are provided with various types of hooks upon 

 their fruits, so that animals such as sheep and cattle and also 

 human beings aid in distributing them, as the hooks become 

 entangled and the fruits carried from place to place ; in this 

 way the seeds are distributed far and wide. A most familiar 

 example of this process is afforded by Cleavers (Klider or Goose 

 grass) {Galium aparine and G. income), the fruit of which is 

 a regular little bur which sticks closely to anything that may 

 touch it. The stems and leaves of this plant are also provided 

 with recurved hooks, by means of which it scrambles and climbs 

 over everything in its neighbourhood. The Enchanter's Night- 

 shade of our gardens is another weed that possesses similar 

 burs. The fruit of the Corn Buttercup (Ranunculus arvensis) is 

 covered with large hooked spines, which make the plant a most 

 troublesome weed, as the seed is so easily distributed. 



(b) Vegetative Reproduction. — Whilst some of the weeds are 

 common because of the large quantities of seed they produce 

 and the special adaptations provided for its distribution, other 

 plants are specially fitted to reproduce themselves vegetatively, 

 increasing by means of certain extensions of the mother plants. 

 Some do this by means of runners of different kinds, throwing 

 out long thin stems bearing clusters of leaves at the joints or 



1 The ovary, when ripe, develops into the fruit, which encloses the seed or 

 seeds In Composite each fruit is single-seeded. 



