462 THE BEAN PLANT. [CHAP. 



warmth and moisture, then germinates. The cotyledons 

 of the contained embryo swell, and burst the see'd coat, 

 but in this species remain underground, and do not 

 become green. The nutritious- matters which they contain 

 are absorbed by the plumule and radicle, the latter of 

 which descends into the earth and becomes the root, while 

 the former ascends and becomes the stem of the young Bean- 

 plant. The apex of the stem retains, throughout life, the 

 simply cellular structure which is, at first, characteristic of the 

 whole embryo ; and the growth in length of the stern, so far 

 as it depends on the addition of new cells, takes place chiefly, 

 if not exclusively, in this part. The growing point does not 

 terminate in a single apical cell, as in the Fern, but con- 

 sists of a number of small, actively dividing cells, termed 

 collectively the apical meristem. The root likewise develops 

 its tissues from an apical meristem, but this, as in the Fern, 

 is protected by a root-cap. 



The leaves cease to grow by cell multiplication at their 

 apices, when these are once formed, the addition of new 

 cells taking place at their bases. Each leaf is compound, 

 the common petiole bearing from four to six leaflets. 



The tissues which compose the body of the Bean-plant 

 are similar, in their general characters, to those found in the 

 Fern, but they differ in the manner of their arrangement. 

 The surface is bounded by a layer of epidermal cells, among 

 which are stomata similar to those described in the Fern. 

 Within the epidermis is a broad zone of tissue, termed 

 the cortex. The greater part of this zone consists of paren- 

 chymatous cells of the usual structure. At the four pro- 

 jecting corners of the stem however, the cortical tissue 

 has a somewhat different structure, consisting of cells which 

 have their walls much thickened at the points of junction. 

 Within the cortex comes the large central cylinder, consisting 



