Olive and Date Plantations in New South Wales. 595 



diversity of opinion held on this point of vegetable physiology. They also 

 furnish good grounds of comparison between the observed facts of M. Mirbel, 

 and the purely theoretical fancies of others. 



We have already stated that M. Mirbel long ago proved the truth of Dr. 

 Grew's ideas ; and also agrees with the corresponding opinion of Duhamel, 

 respecting the cambium being the incipient layer of wood and bark. The 

 latter author's experiments to prove that the cambium proceeded from the 

 bark, and not from the alburnum, are, however, perfectly vague and unsatis- 

 factory. Fixing a thin plate of silver between the liber and the alburnum in 

 spring, to see, at the end of the growing season, on which side of the plate the 

 new deposit of wood would be found, was ingenious ; but, unless the plate 

 were placed with the greatest care, so as to be entirely free from the cambium, 

 no certain result would follow ; because the cambium at that season is so 

 thin and mucilaginous, that it is impossible to separate the bark from the 

 wood without portions of the cambium adhering to both ; in which case no 

 plate, however thin, could be so inserted as to separate the cambium from the 

 wood, nor yet from the liber, and, consequently, the metal a would necessarily 

 be found buried in wood. 



Du Petit Thouars's theory was only a renewal of the old notion of Dr. 



Darwin, who, on seeing new fibres descending from young shoots on the top 



of an old half-rotten willow pollard, concluded that the new layer of wood on 



.all healthy trees was formed in the same manner ; that is, by a tissue of radicles 



which descend from the living spray of the top. 



- But all theories which presuppose a descent of fibres from shoots, buds, or 

 leaves, are invalid for want of proof. The ducts which convey the juices to 

 those expanding organs, are mistaken for fibres descending from them ; and 

 besides, it would appear from the description given of the circumstance, that 

 the fibres are formed before the leaves. All difficulties, however, concerning 

 the constituents of the new zone of wood disappear, if we only admit, with 

 M. Mirbel, that the cambium is the incipient layer of wood and bark ; and, 

 moreover, that it is the vital membrane whence all secretion proceeds. He 

 has proved that it exists in the roots of one monocotyledon : and no doubt 

 it exists in the stems also ; for, when the stem of a Cocos nucifera is split in 

 two, we can trace the fibrous structure of the expanded fronds for a consider- 

 able way down the trunk ; other fronds are rising in their embrace ; these 

 involve others, and others a still younger set, until we arrive at a point 

 in which all visible forms melt, as it were, into a homogeneous substance ; and 

 what is this substance but cambium ? 



Besides, it is now proved beyond a doubt, that cambium, as well as perfect 

 layers of wood, is formed for many years upon the roots of certain trees, after 

 the trunk, branches, and foliage have been dissevered ; showing decidedly 

 that the vital membrane can enlarge itself without assistance from either 

 shoots, buds, or leaves. This is a circumstance which has been witnessed 

 and attested by the celebrated M. Dutrochet, an authority who banishes 

 every kind of doubt. And if so, what then becomes of all our luminous 

 eSsays and lectures on " the elaboration " and " organisability " of the sap ; the 

 descent of fibres from the leaves and buds, &c. ? 



Although M. Mirbel has already said every thing which a practical physi- 

 ologist can advance, to convince every candid mind that the cambium is the 

 incipient layer of wood ; yet it is to be wished that he had adverted to the 

 origin of it more particularly than he has done in the present extracts. 



Chelsea, Sept. 1839, 



Art. V. On (he Olive and Date Plantations in Neiv South Wales. 

 By Dr. Lhotsky. 



The hint which I had thrown out in one of the preceding Numbers of the 

 Gardener's Magazine has been either soon followed, or, perhaps, anticipated, 



