72 TISSUES 



XI. Sense-organs (tactile pits, papillae, hairs and bristles ; statoliths 



organs, such as starch-sheaths or the columellae of root-tips; 



ocelli and other light-perceiving organs). 



XII. Stimulus-transmitting organs and tissues (protoplasmic connecting 



threads: the stimulus-transmitting tissue of Mimosa pudica). 



The system of meristematic tissues is properly placed at the 

 head of the above list, because, on the one hand, it comprises the 

 embryonic conditions of all the permanent tissues, while from another 

 point of view it may be regarded as an independent tissue-system 

 which performs the special function of providing the raw material 

 for the construction of these permanent tissues. In view of their 

 principal functions the dermal and mechanical systems may be 

 grouped together under the head of protective systems. The six follow- 

 ing systems namely, the absorbing, photosynthetic, conducting, 

 storage and ventilating systems, together with the organs of secretion 

 and excretion are concerned with the various aspects of metabolic 

 activity. The last three systems, comprising the structures that serve 

 for the execution of movements and for the perception or transmission 

 of stimuli, are correlated with functions which were at one time con- 

 sidered to be peculiar to, or at any rate distinctly characteristic of, the 

 animal organism ; it is now, however, well known that the supposed 

 distinction between animals and plants based upon the presence or 

 absence of these functions, is valueless, except in special cases. 



Tor the sake of convenience, the consideration of the anatomy and 

 histology of the reproductive organs of plants is usually relegated to 

 the text-books which deal with special morphology and taxonomy. 

 The detailed study of this subject does not in any case come within 

 the scope of the present work. Hence, as regards reproductive organs 

 in general, and the Angiospermous flower and fruit in particular, only 

 those anatomical features will be taken into account which can lie 

 matched in the vegetative region of the plant-body ; in other words, 

 the flower and the fruit will be ignored, except in so far as their 

 structure may help to illustrate the characteristics of one or other of 

 the above-mentioned vegetative tissue-systems. 



