278 MINNESOTA BOTANICAL STUDIES. 



of the archegonium until it comes in contact with the firm nu- 

 cellar tissue {Jigs. 4—p). Many of its cells are often crushed by 

 this backward pressure, but in the mature embryo of the seed 

 the protocormal region is still evident {Jigs, u, J4, j6). 



2. The Blastema. — The blastema arises directly from the 

 small-celled, basal tissue of the protocorm, and invades the 

 gametophyte as abroad, blunt cylinder {Jigs. 4, 5, 6, 8). The 

 central region of the gametophyte seems to be the least resistant, 

 and from the latterly disposed protocorm, the blastema directs 

 its growth into this central tissue {Jigs. 5, 8). The path, which 

 the embryo is to follow, is marked out for a considerable dis- 

 tance ahead of it by disorganized cells {Jig. 8). 



At first the metacormal bud is throughout meristematic ; but 

 very soon there can be distinguished two growth-foci, one directly 

 behind the other, in the axis of the blastema, and only separated 

 from each other by a very few cells {Jig. j). The apical growth- 

 focus is the growing point of the stem, and includes the entire 

 apical region of the young metacorm. The second growth- 

 focus is the growing point of the root. It first becomes notice- 

 able through the apparent diverging from this area of the indefi- 

 nite cell-rows which extend forwards towards the apex of the 

 blastema {Jigs. J, 6). It also immediately begins to cut off 

 rectangular cells from its end towards the protocorm, which 

 form the characteristic, parallel cell-rows of the central root 

 cap region {Jigs. 5-7). Both growing points arise through a 

 localization of growth activity out of one general meristematic 

 tissue ; and hence, from the first are many-celled meristems. 



3. Cotyledons and Leaves. — The cotyledon primordia origi- 

 nate through a localization of growth-activity in the marginal 

 region of the broad apical meristem {fig. 6). They first appear 

 as crescent-shaped mounds of tissue, which push rapidly ahead 

 of the stem apex {Jig. 7). Each cotyledon has an apical 

 meristem. Of the many embryos from Japan examined, all 

 were dicotyledonous with the exception of two ; but among the 

 hundreds of embryos from the Missouri Botanical Garden, which 

 have been studied, over fifteen per cent, were tricotyledons. 

 The more common number of cotyledons is therefore two ; but 

 it is by no means constant as is usually stated to be the case. 

 The plumular leaves, whose origin directly follows that of the 

 cotyledons, arise in the same manner as the latter, but have a 

 much more restricted intraseminal growth {fig. 10). 



