THE DiCOTYLEDONES 



1 641 



ginous. The embryo is generally bent, in some genera in the plane of the 

 cotyledons and in others at right angles to this plane. This difference has 

 also been used in classification. The family is a large one, comprising about 

 220 genera and over 1,900 species. They are found mainly in north 

 temperate regions of both the New and Old Worlds, some extending their 

 range northwards into the sub-arctic. 



Fig. 1504. — Erysimum peiojskiamim. Longitudinal section 



of flower. 



The chief anatomical feature is the presence of characteristic secretory 

 cells which contain myrosin.* The stomata are peculiar in that the guard 

 cells are surrounded by three subsidiary cells, of which one is smaller than 

 the other two. The vessels have simple perforations and there is rarely any 

 medullary ray parenchyma. Epidermal hairs are frequent and very variable 

 in form. Some are multicellular and dendroid and the form of these hairs 

 may be used as a basis of classification. 



There have been a number of attempts to split up this large family into 

 natural units but it must be admitted that each of them is largely artificial. 

 The uniformity of the floral structure makes the discrimination of groups 

 or even of genera a difficult matter. Linnaeus made use of the shape of the 

 fruit as his basis of separation. This view was extended by de CandoUe 

 and later adopted by Hooker in his "Student's Flora". Pomel drew attention 

 to the arrangement of the cotyledons while in the seed, and to the early 

 development of the radicle, and used this character as the basis of his 

 classification, thereafter using the fruit character to split up his three 

 main groups still further. Prantl in the 'Tflanzenfamilien" depended 

 mainly on the presence or absence and the branched or unbranched 



* Myrosin is an enzyme which accompanies the sulphur containing-glucosides in Cruci- 

 ferae and hydrol\ses them to glucose and \arious isothiocyanates (mustard oils). 



