540 History of the Doctrine of [BOOK in. 



particularly struck by the great longitudinal extension which 

 accompanies growth, because, as he says, the vessels still con- 

 tinue hollow, as a glass tube when drawn out to its utmost 

 extent retains its canal. He finds Borelli's idea confirmed, that 

 the young shoot grows by the extension in length of the 

 moisture in the spongy pith ; and he endeavours to explain the 

 fact that the growing shoot does not extend equally in the 

 transverse direction, and so become spherically rounded off like 

 an apple, from the nature of the structure of the cell-tissue. 

 That the air enclosed in the tissue and the sap with it presses 

 into the shoot with sufficient force to produce so great an exten- 

 sion, he thinks is proved by his experiments, which show him 

 the great force with which the water rises in the bleeding vine, 

 and forces itself into swelling peas ; it is known, he says, that 

 water acts with great force when it is heated in a vessel, for 

 water can be driven into the air by heat ; the sap in plants is 

 composed of water, air, and other active ingredients, and makes 

 its way with great force into the tubes and cells, when it is 

 heated by the sun. 



2. The course of the i8th century gradually increased the 

 number of the phytodynamical phenomena, to which physiolo- 

 gists paid more or less attention, and repeated attempts were 

 made to explain them on mechanical principles. These 

 attempts were for the most part unsatisfactory, because move- 

 ments distinct in kind from one another were mixed up 

 together, their dependence on external influences was not 

 distinctly perceived, and the knowledge of the anatomical 

 structure of the parts which exhibited the movements was, 

 owing to the decline of phytotomy, extremely imperfect. 

 Moisture and warmth played the chief part in these explana- 

 tions, but their mode of operation was expressed in the most 

 general terms ; the mechanical processes in plants were des- 

 cribed much in the way in which a person with very indefinite 

 ideas as to the nature of steam and the construction of the 

 inside of a steam-engine might speak of its movements. The 



