GROWTH. 121 



process of the eggs in the true Infusoria, and some other 

 animals is carried on in a far simpler manner. As the 

 life of plants chiefly exhibits itself plastically in opera- 

 tion, the intimations of a pyschological principle are but 

 slight, and only seen in connection with the phenomena 

 of nutrition, or that evident power which presides over the 

 plant, and places in their true combinations and condi- 

 tions those materials, giving to it form, and upon which 

 the activity of every organ is but conditional. This 

 power in a few cases only is seen in such a state of de- 

 velopment and perfection, that its intimations can be 

 placed alongside the intimations of the sensitive life of 

 animals, nevertheless the intimations of design or aim, 

 and even the development of means to attain designs 

 or aims, is evinced pretty generally in connection with 

 vegetable life, and these might alone be sufficient to 

 show us, that we have still to place the plant somewhat 

 nearer to the animal than we have hitherto done. 



GROWTH AND PROPAGATION. 



176. When speaking of stems under the anatomy of 

 plants, we divided them into Dicotyledonous or Exoge- 

 nous, Monocotyledonous or Endogenous, and Acotyle- 

 donous stems ; we have now to inquire into the manner 

 in which they increase in size, and whether they differ 

 in this respect from each other. 



177. We have mentioned that in a section of an 

 Exogenous stem there is visible a central system 

 composed only of cellular tissue, called pith ; next 

 to this, and enveloping it as it were in a case, is the 

 medullary sheath composed of spiral vessels ; following 

 which are layers of woody matter formed of fibre and 

 vascular tissue ; and, lastly, the bark ; and that there 

 run layers of cellular tissue from the centre of the stem 

 to its circumference, called the medullary rays. 



178. Now although this is the general disposition of 

 parts in such a stem, instances are to be met with in 



M 



