On the Anatomy of Vegetables, 429 



from the deleterious influence of meteors, and fhelters it front 

 exceffive heat, cold, moifture, and drought. In a word, it 

 protects it againft all external caufes which might hurt it. 

 Befides this, it ferves for fenfible and infenfible tranfpiration, 

 and the abforption of gas, and of thofe fluids difperfed 

 throughout the atmofphere. For this reafon it is often 

 pierced with very vifible pores : I fay often, becaufe it is not 

 indeed a general law ; and the epidermis of pulpy fruits, for 

 example,"has no apparent pores. I mud add, alfo, that 

 thefe. fruits tranfpire very little, as Dr. Hales has fhown in 

 his Vegetable Statics. 



Chapter IX. 



On the Organizing Svbjlance, or Cambium of DuhameL 

 Hjpothejis on the formation and Development of the Cel- 

 lular TiJJiie and the Tubular Tiffue* 



All the parts of the vegetable have, at firft, been mucilagi- 

 nous and fluid ; and it is only in the fucceffion of time that 

 the tiflue becomes firm and folid. This ftate of feeblenefs is 

 vifible in the feed. The embryo at firft is but a drop of mu- 

 cilage, in which the moft powerful microfcopes difcover no 

 organ This fubftance has a vitreous appearance. It is 

 fpeedily dried and deftroyed by the contact of the air. Pro- 

 perly {peaking, it is not a fluid ; it is an organized fubftance, 

 fimilar to the white of an egg. The organizing fubftance is 

 formed during the whole time of the increafe, and it depofits 

 itfelf in that part of the tiflue where the vegetable ought to 

 acquire the greater): vigour. In the monocotyledons it is 

 placed around each ligneous filament; in dicotyledons it is 

 at the furface of the foft part of the wood and the medullary 

 canal. Every day, therefore, the ligneous filaments of the 

 cotyledons are feen to aflume more volume, and the con- 

 centric ftrata of the dicotyledons to be multiplied, and their 

 pith to be converted into wood. The organizing fubftance is 

 the more abundant, and is renewed with more facility as 

 the individual is younger and founder; as it is in a more 

 favourable fituation, and as the feafon is better fitted for ve- 

 getation. This fubftance infenfibly afTumes determinate forms, 

 whether the fluids develop in it, by their impulfe, the cells 

 and the tubes, or whether an unknown power acts in it 

 alone, and determines thefe developments ; or whether, as is 

 probable, thefe two caufes, united and combined, a& in con- 

 cert to change into membranous tiflue the organizing fub- 

 ftance, it is certain that the vegetable acquires a more con- 

 siderable volume, and that it daily lengthens, and becomes 



Q 2 thicker. 



