408 



MR. MAIN S VEGETABLE PHYSIOLOGY. 



law of vegetable nature ; for where do we find the most insignificant vegetable 

 body come into visible existence without having a pre-existing embryo or nidi- 

 mental atom, whence it derives its essential structure and qualities. There is no 

 such instance in nature. Can the most minute species of Fungi spring forth with- 

 out its propago, or the smallest herb without a seed, or previously existing part of 

 itself? Is the bark or wood self-productive ? No : when either is destroyed it 

 cannot be renewed but by the assistance of that vital member which is the origin 

 of both. 



" Admitting, then, that plants and certain parts of plants possess the property of 

 perpetual reproduction and extension, a question follows : How is this subdivision 

 effected? In the case of bulbs it has already been stated that the radicle plate is 

 composed of an endless train of gems, which are developed in the order of their 

 seniority ; tubers are multiplied by division or branches ; fibrous-rooted herbaceous 

 plants perpetuate themselves by lateral offsets ; but how is the annual subdivision 

 of the vital envelope of trees accomplished ? To this question a direct answer 

 cannot be given, because the process is invisible ; but we can gain a knowledge of 

 the changes which take place between the wood and the liber of a tree by making 

 frequent incisions through the bark, and marking the changes during the spring, 

 summer, and autumn growth. 



" In early spring, say in the beginning of February, the transverse and vertical 

 sections of the stem of four years old appear as represented. 



Transverse and perpendicular sections of a stem four years old ; the latter through the pith. 

 a, pith and wood of the first year; b, c, d, layers of wood of the second, third, and fourth 

 years ; e, the four thin layers of bark. 



" About the end of May, sooner or later, according to the favourableness of the 

 season, similar sections of a stem of the same age will appear thus. 



Sections of a stem as it appears in May or June of the fifth year, 

 the swelling: cambium, 



The white spaces show 



