INTRODUCTION TO CRYPTOGAMIC BOTANY. 37 



than in many other plants.* But though spiral vessels are 

 comparatively unfrequent in the aerial portions, the little pro- 

 cesses which terminate the roots in very many species as first 

 observed by Dr. Hooker, and, indeed, where no such swellings 

 appear, the external cells of the rootlets abound in spirals very 

 much after the fashion of those which are so conspicuous Ui 

 the aerial roots of Orchids, f That such tissue, when young, is 

 very active, there can be no doubt, but after a time, it merely 

 contains air, and sometimes, as in Tricfiia and Hepaticce, an- 

 swers the subordinate purpose of opening and closing the peridia 

 by mere elasticity. And it is possible that some secondary end, 

 as the securing a channel for the passage of air under strong- 

 pressure, may be answered even in plants of tlie firmest struc- 

 ture, in which the yielding of such tubes, and their return to 

 their former calibre, may be of consequence. Some pm-pose 

 of this kind is certainly effected by the tracheae of insects, which 

 are so like to spiral vessels in appearance, as to suggest some 

 similarity of fimction. The functions, however, of spiral vessels 

 in vegetables are so obscure, that we can deduce little from 

 their presence or absence. They cannot be in themselves in- 

 dispensable, or such enormous trunks as are presented by some 

 Conifers covild not exist ; and whatever their function may be, 



* A spiral structure is very visible in the wood cells of the Yew, and, 

 as mentioned above (p. 8), in the white Spruce drift wood of the Arctic 

 Regions. Dr. Hooker has shewn that the close tissue produced at the 

 end of the year is eminently spiral, while the peculiar pine tissue is 

 produced only when vegetation is in full vigour, intimating the latter 

 to be more perfect than the former. See also Berendt Org. Eeste im 

 Bernstein, tab. 2, fig. 7, 12. 



t Specimens of roots of Dacrtjdmm and other Conifers have been 

 long since sent from Australia and New Zealand, laden with these pro- 

 cesses. The specimens were sent as parasitic Fungi, but a very slight 

 examination was sufficient to shew that they were really part and 

 parcel of the root. Dr. Hooker's attention was drawn to them acci- 

 dentally, and he communicated to me his observations on their structure, 

 which I was able comiDletely to confirm. Afterwards we found similar 

 bodies on most of the Conifers cultivated at Kew. I have lately ascer- 

 tained that the structure is figured in Hartig Lehrbuch der Pflanzen- 

 kunde, tab. 18, a woi'k which is apparently in very few hands in this 

 country. 



