Chapter XXVI. 



Poplars and Willows. 



There have now been passed in review the principal Minne- 

 sota types of higher seed-plants in which a single embryonal 

 leaf is produced. In all such forms the first leaf springs from 

 the tip of the nascent plant in the seed, and the rudiment of the 

 stem arises as a lateral protuberance of the young embryo. 



Plants with two-leaved seedlings. The plants included in 

 the lower class are not so numerous as the species which be- 

 long to the other and highest group of the vegetable kingdom. 

 The latter produce a pair of seed-leaves by bulgings at the end 

 of the young spherical embryo shortly after it has begun to 

 form from the fecundated egg. The two seed-leaves are devel- 

 oped opposite to each other and between them is situated the 

 growing point of the stem, so that when seedlings of plants of 

 this group come above ground they may generally be recog- 

 nized by the pair of seed-leaves between which the little bud 

 of the stem gradually unfolds itself. There are, however, some 

 exceptional cases which need attention before passing on. In 

 a few forms the growth of one of the seed-leaves is at a very 

 early stage arrested, so that when in a ripe seed the plantlet is 

 observed it may seem to have but a single seed-leaf. This is 

 true of some bladderworts. Another irregularity is to be no- 

 ticed in the embryos of certain parasitic plants like the mistletoe 

 or dodder. In these the seed matures without the develop- 

 ment of any seed-leaves whatever, and the embryo, upon dis- 

 section of the seed, will be found to exist as a tiny, more or 

 less spherical body near one end. Here, too, it may be recalled 

 that some members of the pine family produce a pair of seed- 

 leaves, yet they would not on that account alone be included 

 in the class now under consideration. In all instances of irreg- 

 ularity it is conceived that special influences must have been 



