24 Origin of the British Flora. 
animals, and part of which is cut off by a fence so that it 
has remained undisturbed. Umbelliferous plants which 
possess burrs, however, behave quite differently. They are 
less tall and springy, and, like other plants with burrs, are 
so arranged as to scrape the burrs against any passing 
animal, but usually not to fling them. 
Many plants have capsules so arranged as to scatter the 
seeds when forcibly disturbed, but not otherwise to drop 
them. The Poppies, Wild Hyacinth, Henbane, and various 
caryophyllaceous plants, have capsules erect in fruit and 
opening above, and the stems become stiff and elastic when 
the seeds are ripe. In some plants such as Evodzum, the 
seed can actually crawl away from the parent. Certain 
trees, such as the Ash, Maple, Hornbeam, and Pine possess 
winged fruits which when detached by a breeze tend to be 
carried short distances, clear of the shadow of the parent, 
though the seed itself is of considerable weight. They com- 
bine in this way the advantages of a large embryo, which 
gives the young plant a copious store of nutriment to draw 
from while it is competing with the short herbage, with a 
seed sufficiently mobile to reach places where it can obtain 
sunshine and new soil. 
The majority of our plants, as already remarked, have 
other means of dispersal, which will enable the species 
occasionally to overleap barriers—a faculty very different 
and probably far more important than the slow spreading 
over short stages that has just been spoken of. Here it 
may be pointed out that this conquest of the land foot by 
foot or yard by yard is insufficient to account for the 
present distribution of our flora. It cannot surmount 
barriers, and will not account for the mode of occurrence 
of such a plant as £rca celzaris, which occupies in profusion 
two compact areas, one in Cornwall and one in Dorset, 
and has every appearance of spreading in each case from a 
