X] ENDODERMIS 185 



The advantage of ventilation here gained is probably greater than the dis- 

 advantage that follows from laying open the conducting tract. 



This exposure of the conducting tracts is carried much further in Opliio- 

 glossum. Here the endodermis is discarded early : the pith dilates with 

 large leaf-gaps, so that the ground-parenchyma appears traversed by unpro- 

 tected vascular strands. The same is the case with the Marattiaceae, though 

 here the number of the strands is much higher, traversing the distended, 

 sappy stock. After the brief juvenile stage is past, there is as a rule no endo- 

 dermal barrier in these Ferns (compare Fig. 128, Chapter Vll). Consequently 

 no such question of the proportion of surface to bulk arises. But on the 

 other hand by discarding the endodermis the conducting tracts have lost 

 that protoplasmic control which the endodermis gives. This state may serve 

 for semi-xerophytic plants, such as the Ophioglossaceae and Marattiaceae, 

 with sappy stocks and leathery leaves, and sluggish fluid-transit. But it 

 would not serve for plants where fluid-transit needs to be rapid, and in 

 particular for those where the leaf-structure is delicate. 



The Leptosporangiate Ferns, which are mostly delicate hygrophytes, 

 comprise the vast majority of the living species of Ferns. They have taken 

 a different course of structural development, in which the endodermal barrier 

 is strictly maintained in its complete form, while intercellular spaces are as 

 a rule absent from their vascular tracts. They show in their peculiar vascular 

 structure to what shifts a plant is put as it increases in size by primary and not 

 by cambial activity, maintaining meanwhile its vascular system under com- 

 plete endodermal control. All of these Ferns start from the protostelic state. 

 It appears from comparison along phyletic lines, parallel but yet distinct, 

 that a disintegrated stelar structure replaced the protostele. The successive 

 steps of this may be seen with varying degrees of clearness in the successive 

 stages of the individual life, those steps appearing as the stele enlarges. 

 According to the reasoning already brought forward, it is on the question of 

 enlargement that the problem of proportion of surface to bulk of the stele 

 turns. The modifications of form of the stele seen in the advanced Lepto- 

 sporangiate Ferns may be held as the means of its solution. 



Since the difference in vascular construction between the more primitive Eusporangiate 

 Ferns and the later Leptosporangiate Ferns turns upon the frequent discontinuity of the 

 endodermis in the one, and its being uninterrupted in the other, it would be well to examine 

 this tissue in detail. In the Filicales two types of endodermis are recognised. That which 

 has been styled /;7>;/<a:;j is characterised by the presence of the well-known Caspary-band, 

 which appears as a rule in the middle zone of the radial walls, though frequently it may 

 lie rather nearer to the central than to the peripheral side of each cell. Since the Caspary- 

 band is suberous, and extends continuously all round each cell, it makes the endodermis 

 impervious to the transit of fluids or soluble substances, except through the protoplasts of 

 the cells, which during life are able to control their passage. Moreover, as the cells are all 

 fitted together so as to form a continuous sheet, even the primary endodermis as well as 



