INVERTEBRATA, CRYPTOQAMIA, MICROSCOPY, ETC. 827 



a. Tliick-wallcd parenchyma and bast are simply stretched, and 

 then ruptured. 



h. The epidermis and collenchymatous cells continue to grow 

 for a time together with the root, and are only subse- 

 quently overtaken and broken through by it. 



3. In consequence of the increase in thickness, a union in growth 

 takes place between the root and the adjoining tissue of the mother- 

 organ, when the latter is still in a formative condition. 



4. The subsequent increase in length of the cells of the root 

 causes also the innermost cortical cells of the mother-organ, which 

 are in anatomical connection with the root, to become stretched 

 radially. At the same time the true increase in thickness causes the 

 cortical cells to form curves parallel to the surface of the root. In 

 consequence of the similarity of the cells which form the curves, 

 curves crossing each other at right angles are also seen ; and hence 

 the root appears to run out from a broad base, while in fact it is 

 considerably contracted below. 



Influence of Direction and Strength of Illumination on certain 

 Motile Phenomena in Plants.* — E. Stahl's paper on this subject 

 contains the following general observations : — 



The effects of light in this respect are very various. Sometimes 

 formed cell-contents, as chlorophyll-grains, in the interior of the 

 protoplasm, are set in motion, and carried within the cell-cavity 

 to places which indicate a definite relation to the direction of the 

 rays of light. In other cases the influence of the light is exhibited, 

 not in the direction of certain particles, but of entire free motile 

 organs. 



In spite of differences in particular cases, a general and important 

 phenomenon is evident, that, when other conditions are the same, 

 csijecially where the direction of the light remains the same, the 

 variations in sensitiveness to light depend entirely on the intensity of 

 the light. 



When the direction remains the same, the chlorophyll-plate of 

 Mcsocarpus places itself at right angles to this direction when the 

 illumination is weak ; but, when the intensity passes a certain limit, 

 the plate turns througli an angle of 90^, and places itself in the 

 direction of the rays. A swarnispore usually turns its anterior end 

 to weaker light, the reverse when the light is stronger. This is true 

 both in the case of jiositively heliotropic filaments and of those which 

 grow at right angles to the direction of the light. The behaviour of 

 species of Chisterium varies with the intensity of the light ; and the 

 same is true for diatoms, and, according to older observations, for 

 Oscillatorie;e and Myxomycetes. 



The varying susceptibility of vegetable protoplasm to the influence 

 of light, whicli has tliiis been determined in a number of single cases, 

 is of j)r()portionate inq»ortance in determining the positions dependent 

 on light of more complicated organs, as is illustrated in the case of 

 Vauchcria. Whoa a filament of Vancheria is illuminated from one 



• '13ot. Zeit.,' xxxviii. (1880) p. 2'J7. 



