20- 



may be evolved again, by tbe second parcel, on the other side. Certain plants 

 may absorb some * poisons' by their roots, with impunity, which would be destruc- 

 tive to others. Other phenomena illustrate and confirm these truths, and it 

 would be altogether superfluous to detail them. 



But irrespective of the facts connected with the excretions of the roots which 

 have been assumed as explanatory of the necessity of the rotation of crops, there 

 is another interesting question involved in the curious inquiry, to which I am 

 desirous to call attention, and which, as far as I know, has never been once sus- 

 pected. It is this : how far particular plants may, or may not, prove injurious by 

 their proximity to others, from exudations and exhalations of a more or less vola- 

 tile kind, as well as gaseous products arising from stems, foliage, and flowers ; and 

 therefore to what extent plants reciprocally affect each other. Certain plants grow 

 freely side by side, or in juxta-position ; whilst the very reverse is the fact with 

 others. Certain shrubs luxuriate beneath the shade of trees, and the copious 

 showers that trickle from their branches ; while myriads would be destroyed 

 under similar circumstances. Many plants perish near others, or disappear 

 without any visible cause. Though the corrosive liquid that distils from the 

 branches of the Manchineel is of too palpable a character to be questioned, there 

 are others that seem more dubious. The blighting influence of the Barberry on 

 certain crops, however, appears not to be apocryphal. The hardiest weed will not 

 dare to shew itself beside that gigantic reed, the Bamboo ; and bees fall down 

 dead suddenly, should they perchance alight on the branches of the Rhus vernix. 



J. MURRAY, F.L. & G.S. 



(To be continued.) 



AN ACCOUNT OF THE STRUCTURE OF THE HEART IN THE 

 TESTUDO MYDAS, OR GREEN TURTLE. 



The heart, in the several families of the Tortoises and Turtles, presents 

 curious peculiarities adapted to the mode of life of the animals in whom these ano- 

 malies of anatomical disposition are met with. Each species varies a little in the 

 anatomical structure of the central organ of the circulation ; but I shall, in this 

 paper, take the Testudo mydas as the type of all animals of this order. The 

 Testudo mydas, or Green Turtle, — the Tortue franche, of Cuvier, called by the 

 Germans, die Griine Schildkrote, — is found on all, or most of, the coasts of the 

 torrid zone, feeding upon the weed at the bottom of the ocean, approaching the 



