THE STEM OF VASCULAR PLANTS, 153 



of seeing a distinguished observer arrive at the same con- 

 clusions, and support them by a very curious fact. Du 

 Petit-Thouars has remarked that the Lecythis, which is 

 certainly an Exogenous plant, germinates without ap- 

 parent cotyledons,* but that its first shoot has a very 

 large pith, which serves to nourish the young plant, and 

 thus performs, physiologically, the function of the coty- 

 ledons, as the ordinary pith does for the buds. After it 

 dries up (if it be of any use, which is very doubtful) it 

 can only, as Grew has thought, become a receptacle of 

 atmospheric air. In the article upon the Medullary 

 Rays, we shall speak of the analogy of the pith with the 

 cellular envelope, and the rays which unite these two 

 organs. 



All that we have said of the Central Pith, applies, 

 with slight differences, to those of following years, each 

 of which, under the form of a Medullary Zone, repre- 

 sents the central one ; it results from their position that 

 their form is very different, but their analogy of nature 

 is perceptible. We may be assured of this by examining 

 certain trees, such as the Rhus Typhinum, in which the 

 pith is coloured, and where we observe a similar colour- 

 ing to that of the centre in those of following years. 

 These last are never broken as the centre one, owing to 

 their position ; but they follow, moreover, the same 

 changes ; at first they are fresh and full of juice, and 

 finish by drying up or solidifying. When a small piece 

 of wood is macerated, its cellular parts, never being of the 

 same density as the fibrous zones, are first changed, and 

 the latter are then obtained, more or less detached from 

 one another, by the disappearance of the intermediate 

 cellular tissue. 



* The true cotyledons of this genus appear to be so intknately joined 

 together that they cannot be separated, and form an undivided embryo, 

 similar to that of Monocotyledons. (See Book III. chap. iv. sect, iv.) 



