210 COMPOUND ORGANS OF PLANTS. 



this that the radicle of the embryo always points to the micro- 

 pyle. This fact the student may readily ascertain by dissect- 

 ing any seed which has a visible micropyle on the episperm, 

 and ascertaining the direction of the radicular extremity of the 

 embryo. He will find it invariably pointing to this very spot. 



Sometimes the micropyle entirely disappears from the sur- 

 face of the episperm. Its place, however, may be readily 

 ascertained. If the skin of a seed be carefully examined it 

 will be usually found to be marked with lines or bands which 

 run upwards from the hilum. These lines always converge 

 and meet in the micropyle, so that by following them with 

 the eye, the micropyle may be frequently discovered on the epi- 

 sperm, when owing to its minuteness it would otherwise escape 

 detection. 



The chalaza is more or less visible in 'all anatropous seeds, 

 being often colored and of a denser texture than the surround- 

 ing tissue. At the apex of the seed of the orange and many 

 other plants, it may be perceived on the episperm in the form 

 of a large brown spot. In orthotropous and campulitropous 

 seeds the chalaza is directly superposed on the hilum, with 

 which it is immediately confluent, but in all anatropous seeds 

 it is placed apart from the hilum, and is connected with a vas- 

 cular bundle called the raphe, which forms a longitudinal pro- 

 minence more or less conspicuous on the episperm. In most 

 plants the raphe consists of a single line, as in the castor-oil 

 plant, but in the orange and lemon it ramifies upon the surface 

 of the episperm. Fig. 104. 



It is proper to remark here, that the terms orthotropous, 

 campulitropous, and anatropous, employed to designate the 

 different kinds of ovules, are equally applicable to seeds, all 

 seeds occurring under one or other of these three leading 

 forms. 



