THISTLES. 



107 



of seeds ; or, to speak more correctly, only those plants in the long 

 run succeed in surviving which happen to possess some such facility 

 for constant rotation and occupation of fresh districts. 



It is very interesting in this respect to compare the devices for the 

 distribution of their seeds in some of the thistle's own nearest and best- 

 known relations. The burdock, for example, is in flower and fruit al- 

 most a thistle, though it differs considerably from the thistles proper 

 in its large, broad, heart-shaped foliage. But the burrs, or ripe flower- 

 heads, instead of being surrounded, thistle-fashion, by a very defensive 

 prickly involucre, have developed instead hooked points to their bracts, 

 which catch at once at the wool of sheep, the legs of cattle, and the 

 dresses or trousers of wayfaring humanity. In this way the entire 

 head of seeds gets carried about from place to place, and rubbed off at 

 last against a hedge or post (at least by its unwilling four-footed car- 

 riers), where it forms the nucleus of a fresh colony, and starts in life 

 under excellent auspices, especially if dropped (as it is apt to be) in 

 the immediate neighborhood of a well-manured farm-yard. Hence the 

 burdock has no further need for the down which it inherits, like all its 

 tribe, from some remote common ancestor ; it has substituted a new 

 and more practically effective system of transport en bloc, for the old 

 general composite mode of dispersal in single seeds by a feathery float- 

 ing apparatus. Accordingly, the pappus, or ring of down, though it 

 still exists as a sort of dying rudiment on each fruitlet of the burrs, is 

 reduced greatly in size and expansion, and consists of a mere fringe of 

 short, stiff hairs, useful perhaps in preventing flies from laying the eggs 

 of their destructive grubs upon the swelling seeds. In the common 

 knap-weeds, again, which wait for a high wind to shake out their seeds 

 from the head, this dwarfing of the down has proceeded much further, 

 so that at first sight a careless observer would never notice its exist- 

 ence at all : but if you look close at the ripe fruit with a small pocket 

 lens, you will observe that it is topped by a ring of very minute, scaly 

 bristles, occasionally mixed with a few longer and hairier ones, which 

 are all that now remain of the once broad and feathery down. Among 

 the true thistles, on the other hand, which trust entirely to the gentle 

 summer breezes for dispersal, and which float away often for miles to- 

 gether, innumerable gradations of featheriness exist, some species hav- 

 ing the down composed of long, straight, undivided hairs ; while in 

 others of a more advanced type it consists of regular feathered blades, 

 barbed on either side with the most delicate beauty. Almost all our 

 commonest and most troublesome English thistles belong to this last- 

 named very feathery type, whose seeds are, of course, enabled to float 

 about on the wind far more readily and to greater distances than the 

 simple-haired varieties. 



The thistle pedigree is a long and curious one. The group forms, 

 apparently, the central and most primitive existing tribe of the com- 

 posite family, and it bears in its own features the visible marks of a 



