THE MEGASPORANGIUM AND SEED 



227 



of a hard inner layer known as the sclerotesta and a softer outer 

 one which contains mucilage canals and to which the name of 

 sarcotesta is applied. Both sclerotesta and sarcotesta are pro- 

 vided with a system of ribrovascular strands, but tracheary ele- 

 ments of any kind 

 are absent in the 

 region of the nucel- 

 lus or megaspo- 

 rangium. 



The type of 

 seed delineated in 

 connection with 

 the foregoing para- 

 graph is not the 

 only one charac- 

 teristic of more 

 ancient plants. In 

 Fig. 1 68 is shown 

 another category 

 of seed which, 

 although present- 

 ing the general 

 features of the 

 Paleozoic type, is 

 characterized by 

 certain interesting 

 and important 

 peculiarities. The 

 integument in the 

 diagram is repre- 



FIG. 1 68. Diagram of an ancient type of seed with 

 tracheary mantle surrounding the gametophyte (modified 

 after Oliver). 



sented as consisting of an outer softer sarcotesta and an inner 

 resistant sclerotesta. It incloses, as in the other type, the mega- 

 sporangium or nucellus, and this is likewise provided with a 

 pollen chamber. The only important difference between the seed 

 under discussion and that described in the preceding paragraph 

 is the distribution of the fibrovascular bundles. In the seeds 

 of the first type the tracheary strands are present in both 



