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POPULAIt SCIENCE REVIEW. 
reference to figures, but perhaps this is too much to expect of the author at 
his advanced age. — We may add with satisfaction that the Rev. M. J. Berke- 
ley has commenced a work which will supplement the present, and include 
all extra-European species of Hymenomycetes, and thus render complete 
what the illustrious Swedish mycologist has only in part accomplished. No 
one is so competent to undertake this as the authority who has taken it in 
hand, and we anticipate that in little more than twelve months, if health 
should be given him, we shall be in possession of the new Epicrisis. 
Do Varieties wear out ? — This important question is asked and answered 
in an able article in " Silliman’s American Journal ” for February 1875, by 
Dr. A. Gray, the eminent American botanist. He says there is a philo- 
sophical argument which tells strongly for some limitations of the duration 
of non-sexually-propagated forms, one that probably Knight never thought 
of, but which we should not have expected recent writers to overlook. When 
Mr. Darwin announced the principle that cross-fertilization between the 
individuals of a species is the plan of nature, and is practically so universal 
that it fairly sustains his inference that no hermaphrodite species continually 
self-fertilized would continue to exist, he made it clear to all who apprehend 
and receive the principle, that a series of plants propagated by buds only 
must have weaker hold of life than a series reproduced by seed. For the 
former is the closest possible kind of close breeding. Upon this ground such 
varieties may be expected ultimately to die out ; but “ the mills of the gods 
grind so exceedingly slow,” that we cannot say that any particular grist has 
been actually ground out under human observation How and why 
the union of two organisms, or generally of two very minute portions of 
them, should re-enforce vitality, we do not know and can hardly conjecture. 
But this must be the meaning of sexual reproduction. The conclusion of 
the ’matter from the scientific point of view is, that sexually propagated 
varieties, or races, although liable to disappear through change, need not 
be expected to wear out, and there is no proof that they do ; but, that non- 
sexually propagated varieties, though not liable to change, may theoretically 
be expected to wear out, but to be a very long time about it. 
Transpiration of Plants . — The u American Naturalist ” [Feb. 1875], (in a 
note furnished, if we do not mistake, by one of our contributors to the 
present number) states that Dr. W. R. McNab, of Dublin, has performed a 
fresh series of experiments on the rate of motion of the sap in plants, and the 
transpiration of water from the leaves. The plants selected were the cherry- 
laurel ( Prunus laurocerceus), elm, and privet j and the results obtained were 
as follows : 1. That, under favourable circumstances, a rate of ascent of forty 
inches per hour can be obtained. 2. That, contrary to the generally re- 
ceived opinion, direct experiment has shown that the upward rapid current 
of water does not cease in the evening. 3. That checking the transpiration 
for a short time by placing the branch in darkness does not materially impede 
the rapid current of water. 4. That the removal of the cortical tissues 
does not impede the rapid current in the stem, which moves only through 
the woody (xyleus) portion of the fibro-vascular bundles. 5. That a well- 
marked rapid flow of fluid will take place in a stem after the removal of the 
leaves. G, That fluid will rapidly flow downwards as well as upwards in 
the woody (xyleus) portion of the fibro-vascular bundles, as seen in a branch 
