234 ZOOLOGICAL PHILOSOPHY 



excited by external influences, we shall be convinced that in this kind 

 of living bodies life can only have a feeble activity, even in times and 

 climates when vegetation is rapid ; and consequently that their com- 

 plexity of organisation is necessarily confined within very narrow limits. 



Infinite pains have been taken to become acquainted with the details 

 of plant organisation : search has been made in them for peculiar 

 or special organs of the same kind as some of those known in animals ; 

 and the results of all these researches have done no more than show 

 us that the containing parts consist of a more or less compressed cellular 

 tissue, with elongated cells that communicate with each other by 

 pores and by vascular tubes of various shape and size, mostly having 

 lateral pores or sometimes clefts. 



All the details ascertained on this subject furnish little in the way 

 of clear general ideas, and the only ones which we need recognise are 

 as follows : 



1. Plants are living bodies with less perfect organisation than 

 animals and with less active organic movements ; their fluids move 

 more slowly and the orgasm of the containing parts is very faint ; 



2. They are essentially composed of cellular tissue ; for this tissue 

 is to be found in every part of them, and indeed it is found almost by 

 itself and with very shght modifications in the simplest of them 

 (algae, fungi and probably all the agamous plants) ; 



3. The only change undergone by cellular tissue in monocotyledons 

 and dicotyledons as a result of the fluids moving within them, consists 

 in the transformation of certain parts of this cellular tissue into 

 vascular tubes of varied size and shape open at the extremities, and 

 mostly having lateral pores. 



Let me further add that since the movement of fluids in plants is 

 either upwards or downwards their vessels are naturally almost always 

 longitudinal, and approximately parallel to one another and to the 

 directions of the stem and branches. 



Lastly, the outer part of that cellular tissue, which constitutes the 

 bulk of every plant and the matrix of its low organisation, is squeezed 

 and compressed by the contact, pressure and collision of the environ- 

 ment, and is thus thickened by deposits and transformed into a general 

 integument ^ called bark which is comparable to the skin of animals. 

 Hence we may understand how the external surface of this bark, 

 being even more disorganised than the bark itself by the causes named, 



* If the stems of palms and some ferns appear to have no bark, it is because they 

 are only elongated root collars, the exterior of which shows a continuous succession 

 of scars left by old leaves that have fallen ; this prevents the possibility of a con- 

 tinuous or uninterrupted bark ; but it cannot be denied that each separate part of 

 this exterior has its special bark, although more or less imperceptible on account of 

 the small size of these parts. 



