VEGETABLE PHYSIOLOGY. 27 



adds to their length; and being unable to support their own 

 weight in a straight line, they assume the spiral form. Willde- 

 now ascribes a considerable influence to the current of air in 

 determining the direction of the tendril. 



Of the Sap vessels and Sap. 



Much contrariety of opinion has existed among physiologists 

 concerning the vascular system of plants, and the nature of the 

 propulsion of the sap through their stems and branches. Indeed 

 it is a subject upon which very erroneous ideas have prevailed. 



That the whole vegetable body is an assemblage of tubes and 

 vessels is evident to the most careless observer ; and those who 

 are conversant with the microscope, and books relating to it, 

 have frequent opportunities of observing how curiously these 

 vessels are arranged, and how different species of plants, 

 especially trees, differ from each other in the structure and 

 disposition of them. Such observations, however, if pursued no 

 further, lead but a little way towards a knowledge of the 

 wonderful physiology of vegetables. 



That plants contain various substances, as sugar, gum, acids, 

 odoriferous fluids and others, to which their various flavors and 

 qualities are owing, is familiar to every one ; and a little reflec- 

 tion will satisfy that such substances must each be lodged in 

 proper cells or vessels to be kept distinct from each other. 

 They are extracted, or secreted from the common juice of the 

 plant, and called its peculiar or secreted fluids. Various 

 experiments and observations prove also that air exists in the 

 vegetable body, and must likewise be contained in appropriate 

 vessels. Besides these, we know that plants are nourished and 

 invigorated by the agency of water, which they readily absorb, 

 and which is quickly conveyed through their stalks and leaves, 

 no doubt by tubes or vessels on purpose. Finally, it is observable 

 that all plants, as far as any experiment has been made, contain 

 a common fluid, which at certain seasons of the year is to be 

 obtained in great quantity, as from the vine branches by wounding 

 them in the spring before the leaves appear, and this is properly 



