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58 



MR. N. E. BROWN : KEW AND OLD 



the best of my knowledge there is no other genus in the Vegetable or 

 Annual Kingdoms in which the evohition of species cun be so well demon- 

 strated as bj a collection of living ])hints of Mesemhryanthemiuu, I know of 

 no other genus in which a fairly complete series of adult forms can be found 

 in it ranging from those which, except in size, scarcely differ in form from 

 the cotyledonary stage, into other vegetative types, such as bushes and plants 

 with long trailing stems that are utterly ditlerent from the cotyledonary or 

 embryonic form. And even if it were claimed that some of the groups into 

 which this genus is at present divided really reprcseni distinct genera, it 

 would not affect the obvious chain of evolution in any way. It is not my 

 intention to deal with this point now, but I hope to do so on another 



occasion, 



A point of interest that I do not remember to have seen mentioned by 

 previous authors is the manner in whicli the leaves of some of the sjiecies 

 of this remarkable genus, which only produce 2-4 leaves on each growth in a 

 year, vary in form and size at diil^erent periods^ sometimes in such a marked 

 deo-ree that the same individual ^^^i^n at one season might easily be thought 

 to be a different species when seen at another time of the ycar^ by reason of 

 the two pairs of leaves which form tlie season's growth being more or less 

 dissimilar. This feature is most evident perhaps in the sections Mardda^ 

 Moniliformla^ and Rostrata. AVhen a plant commences to make its new 

 growtli at first only one kind of leaf is seen^ then when the second pair is 

 produced they are often more or less unlike the first pair, so that two forms 

 of leaf may be seen upon tlie same growth. A typical example of this is 



1 



.'ei>resentcd by M. candidn^.'^itntfni 



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Finally, when the 



older pair has withered or fallen away, only one type of leaf will be noticed, 

 which in the case of the Marcida and ALoniliforuda groups will be the pairs 



that form the cones. 



Variation in size is probably due to the varying amount of moisture and 

 food the plant obtains ; for not only does a little more water in the soil or 

 atmosphere induce increase of size^ and may even cause the leaves to crack 

 or burst open, but I have also found that a change of soil will sometimes 

 bring about a similar result. This is well demonstrated by the two figures 

 of M, proAmum on PL 5. Fig. 3 represents a branch with a cluster of three 

 growths upon it cut from a plant and rooted in the autumn of 1917. In 

 May 1918 it was repotted into a different soil, and at that time was very 

 little larger than represented by fig- .3; but hy October of that year all the 

 cones had very greatly increased in size, fig. 4 representing in outline one 

 of the largest a w^eek before it commenced to burst and display its component 

 pair of leaves as described below. Yet in spile of the increase in size I 

 believe it had less water than the plant from which the branch was cut and 



which did not make larger cones than those represented by fig* 3. 



Another pohit of interest is the mode of growth of the Splarroidea^ MonUl- 



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