234 Minnesota Plant Life. 



brought to bear upon the embryo plantlet to modify it from the 

 ordinary type; but when the other structures of the plant are 

 taken into account there is usually no difficulty in assigning it 

 to its proper class. 



Stem structure. A very constant character of plants of the 

 highest class is the presence in the stem of fibres in which a 

 longitudinal layer of cells remains without finally maturing. In 

 trees, the seedlings will be found to have at first a little circle 

 of such fibres, which by mutual pressure upon each other be- 

 come blended together around the central pith. The layer of 

 cells known as the cambium in each of the fibres thus joins with 

 the layer of a fibre next to it and a cylinder of cambium origi- 

 nates, inclosing the young wood and inclosed by the young 

 bark. This layer of cells ordinarily gives rise, during every year, 

 to new wood tissue and new bark tissue. The oldest parts, there- 

 fore, of a tree-trunk, are the outside where one can lay one's 

 hand, and the heart-wood. The bark is younger as one cuts 

 from the outside toward the wood, but the wood is younger 

 as one cuts from the centre toward the bark. In a tree there 

 are just as many bark rings as there are wood rings, but the 

 outer bark is constantly sloughing off, being cracked by the ex- 

 pansion of the wood within and by exposure to the disintegrat- 

 ing effects of the weather. Besides this, the ring of bark that is 

 formed by the cambium during a year is commonly not so thick 

 as the ring of wood. For these reasons bark may be only an 

 inch or more thick, while the wood from circumference to cen- 

 tre may measure even several feet. 



Herbs and trees. To non-botanical observers it is sometimes 

 difficult to explain that there is no very essential difference in 

 structure between an herb and a tree, providing they both belong 

 to the same general family of plants. The elm is so very much 

 larger than the nettles which grow in its shade that upon 

 any one who considers the two together it naturally makes a 

 very different impression. Yet, nevertheless, the general plan 

 of structure in the two is very similar. In the nettle there is a 

 disposition of woody tissue inside, with bark tissue outside, just 

 as in the elm, but the tree persists possibly for hundreds of years, 

 thickening its trunk with every season, while the stem of the 

 nettle dies at the end of the first season of growth. The proper 



