ON THE ORIGIN OF STRUCTURES IN PLANTS 



W. A. CANNON 

 Desert Botanical Laboratory 



The systems of organs of which a higher plant more 

 especially is composed generally hold an intimate phys- 

 ical relation to one another. They are bonnd together 

 so intimately by reason of their position in root or shoot 

 that the growth, development or response to stimulus of 

 one is in a very great measure molded by the growth, 

 development or reaction of all the rest. In addition to 

 these considerations, when the origin of tissues or of or- 

 gans is being investigated, account must also be taken of 

 the nutrition of the special organ as well as its especial 

 relation to environment external to the plant of which it 

 is an integral part. Thus the complex physical interrela- 

 tions, and the physiological correlations' as well, make 

 the study of the functions, and behavior of the individual 

 tissue, or organ, as a possible independent unit one of 

 great difficulty. These general facts probably hold for 

 plant tissues as a whole, but one system, namely, the 

 trichomal system, offers a favorable field in which to 

 study the origin, development and biological relation- 

 ships of plant organs, inasmuch as it is comparatively 

 little affected by other tissue systems. Beyond growing 

 out of epidermal cells, remaining permanently attached 

 to the epidermis, and deriving nourishment from the sub- 

 jacent cells, the trichomes lead an independent existence, 

 and in origin, development and form are not directly in- 

 fluenced, as the other tissues are affected, by the pressure 

 of enveloping tissues, and in certain plants, as Franseria 

 dumosa, the trichomes go one step further on the road 

 to independence, in that they are chlorophyll-bearing and 

 in a sense probably auto-trophic. For these and other 

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