OF THE FERNS AND FLOWERING PLANTS. 
157 
which he has already gathered the flower. The same 
foresight must be extended to the fruit. The latter, indeed, 
is not indispensable, though certainly desirable ; for the 
reader may easily picture to himself what confusion and 
errors may possibly arise, where there is no certainty of 
the examples, which lie together in the herbarium, having 
been the produce of the same plant. 
The above remarks refer with tenfold force to the 
Willows, which seem to have a peculiar facility for hybri- 
dising ; and, therefore, the greatest care should be taken 
to isolate every specimen, and if possible to have it in one’s 
power to identify the very tree from which each was taken. 
The Ferns are no exception to the rule, which demands 
that the plant should be seen in its integrity when dried. 
The crown and root must always, if possible, be secured as 
well as the frond ; and of the latter, those which have no 
fruit on them must not on that account be passed by, as 
the two kinds often exhibit wide differences in form, and 
mark the character of the plant. More than one species of 
the remarkable genus, Equisetum, is furnished with both 
sterile and fertile fronds ; both of which must of course be 
gathered and laid side by side in the herbarium. In the 
case of the common Equisetum arvense, the succulent, 
fawn-coloured, fruit-bearing stem rises upright from the 
soil weeks before the harsh green procumbent frond 
spreads itself over the ground. In others again the fertile 
shaft is entirely unbranched, while the sterile stems are 
enriched by frequent whorls of elegant pendant branches. 
The two sorts of frond may be easily recognised ; while 
the barren stem tapers gradually to a point, the fertile is 
furnished with a stout clavate head, which is in fact the 
receptacle, and contains the spores in a number of separate 
sporangia. These spores are themselves very interesting 
objects : each is furnished with four filamentous processes, 
known as elaters, though very unlike the elaters which are 
mingled with the spores in the capsules of the Hepaticse. 
They are extremely sensitive to the influence of moisture, 
