Comparative Physiology. 567 



It is not likely that the more perfect exogenous stem would 

 be without an aerating apparatus also ; and the dotted ducts, 

 from their great abundance and being more open at the ends, 

 are probably the vessels destined for that purpose. They have 

 been reckoned cellular, not vascular, by some, though others con- 

 sider them vascular ; but it is not easy to point out the limit 

 between cells and vessels, and their lengthened form and dotted 

 sides seem to indicate a vascular nature. When they are 

 wanting, as in pines, their place is probably supplied by the 

 large glandular tubes of the woody fibre. The apparent neces- 

 sity also for an apjjaratus of the kind would seem to point out 

 their office. The necessity of oxygen to stimulate the vital 

 fluid, though not so great in plants as in animals, is yet found 

 to be indispensable, no plant thriving unless both the air 

 and water of the soil contain it ; stagnant air and stagnant 

 water being injurious to both leaves and roots. Other gases are 

 probably introduced by the same means. In the higher animals, 

 the aeration being confined to the lungs, there is more necessity 

 for the fluids being returned, and a greater circulating power is 

 required. In plants, where the vital force is principally confined 

 to the young shoots, the aerating system is more perfect there ; 

 but, being perforated like insects with air-vessels which aerate 

 through the whole extent of the young tissue, the sap circula- 

 ting through the nutrient vessels is not returned again in such 

 quantity to the leaves as the blood to the lungs. Through- 

 out the whole circulation, however, there is a mixture of de- 

 scending and ascending sap. The circulation in insects is also 

 so feeble that it was at one time denied to exist. 



On Circulation in Vegetables, he says : — " In the lower alg£e, 

 entirely cellular in their structure, there appears so little com- 

 munication between the parts, that if a portion be suspended 

 out of the water it will die, while the portion immersed con- 

 tinues to live. Among lichens or wherever there is an approach 

 to a stem, the cellules are found to elongate. In mushrooms 

 the nutriment received by the radical fibres at the base of the 

 stem is transmitted by its elongated cells, and probably by the 

 intercellulary spaces, to the expansion on its summit, where it is 

 difllised in every direction. In the classes where a complete 

 stem is developed with radical fibres and veined leaves, the 

 cellular tissue of the stem and veins of the leaves is elongated 

 so as to resemble fibro-vascular tissue; and there can be no 

 doubt the circulation is through this channel, as stomata have 

 lately been found on them. It appears that where there is no 

 tendency to prolongation in a particular direction, the round 

 vesicles of cellular plants transmit fluid with equal readiness 

 towards all sides ; but, wherever the function of absorption 

 begins to be restricted to part of the surface, there is a tendency 



